feat: Implement common modules

core/src/main/java/com/opencontext/common/CommonResponse.java

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+package com.opencontext.common;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import io.swagger.v3.oas.annotations.media.Schema;
+import lombok.Getter;
+
+import java.time.LocalDateTime;
+
+/**
+ * Standard API response structure class.
+ * Handles success and failure responses in a consistent format to simplify client response processing.
+ * 
+ * This class should only be created through static factory methods,
+ * preventing direct constructor usage to ensure API response consistency.
+ */
+@Schema(description = "Standard API response structure")
+@Getter
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public class CommonResponse<T> {
+
+    @Schema(description = "Request success status", example = "true")
+    private final boolean success;
+
+    @Schema(description = "Response data (null on failure)")
+    private final T data;
+
+    @Schema(description = "Message to display to user", example = "Request processed successfully.")
+    private final String message;
+
+    @Schema(description = "Error code (null on success)", example = "VALIDATION_FAILED")
+    private final String errorCode;
+
+    @Schema(description = "Response creation timestamp", example = "2025-08-07T12:00:00")
+    private final LocalDateTime timestamp;
+
+    /**
+     * Private constructor. Only allows instance creation through static factory methods.
+     */
+    private CommonResponse(boolean success, T data, String message, String errorCode, LocalDateTime timestamp) {
+        this.success = success;
+        this.data = data;
+        this.message = message;
+        this.errorCode = errorCode;
+        this.timestamp = timestamp;
+    }
+
+    /**
+     * Static factory method for creating success responses.
+     */
+    public static <T> CommonResponse<T> success(T data) {
+        return new CommonResponse<>(
+                true,
+                data,
+                "Request processed successfully.",
+                null,
+                LocalDateTime.now()
+        );
+    }
+
+    /**
+     * Static factory method for creating success responses with custom message.
+     */
+    public static <T> CommonResponse<T> success(T data, String message) {
+        return new CommonResponse<>(
+                true,
+                data,
+                message,
+                null,
+                LocalDateTime.now()
+        );
+    }
+
+    /**
+     * Static factory method for creating error responses.
+     */
+    public static <T> CommonResponse<T> error(String message, String errorCode) {
+        return new CommonResponse<>(
+                false,
+                null,
+                message,
+                errorCode,
+                LocalDateTime.now()
+        );
+    }
+}

core/src/main/java/com/opencontext/common/PageResponse.java

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+package com.opencontext.common;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import org.springframework.data.domain.Page;
+
+import java.util.List;
+
+/**
+ * Paginated list response DTO.
+ * Converts Spring Data Page objects into a client-friendly format.
+ */
+@Schema(description = "Paginated list response DTO")
+@Getter
+@Builder
+@NoArgsConstructor(access = AccessLevel.PROTECTED)
+@AllArgsConstructor
+public class PageResponse<T> {
+
+    @Schema(description = "Data list for current page")
+    private List<T> content;
+
+    @Schema(description = "Current page number (zero-based)", example = "0")
+    private int page;
+
+    @Schema(description = "Number of items per page", example = "10")
+    private int size;
+
+    @Schema(description = "Total number of items", example = "100")
+    private long totalElements;
+
+    @Schema(description = "Total number of pages", example = "10")
+    private int totalPages;
+
+    @Schema(description = "Whether this is the first page", example = "true")
+    private boolean first;
+
+    @Schema(description = "Whether this is the last page", example = "false")
+    private boolean last;
+
+    @Schema(description = "Whether there is a next page", example = "true")
+    private boolean hasNext;
+
+    @Schema(description = "Whether there is a previous page", example = "false")
+    private boolean hasPrevious;
+
+    /**
+     * Static factory method to convert Page object to PageResponse DTO.
+     */
+    public static <T> PageResponse<T> from(Page<T> page) {
+        return PageResponse.<T>builder()
+                .content(page.getContent())
+                .page(page.getNumber())
+                .size(page.getSize())
+                .totalElements(page.getTotalElements())
+                .totalPages(page.getTotalPages())
+                .first(page.isFirst())
+                .last(page.isLast())
+                .hasNext(page.hasNext())
+                .hasPrevious(page.hasPrevious())
+                .build();
+    }
+
+    /**
+     * Static factory method to convert Page object with custom content to PageResponse DTO.
+     */
+    public static <T> PageResponse<T> from(Page<T> page, List<T> content) {
+        return PageResponse.<T>builder()
+                .content(content)
+                .page(page.getNumber())
+                .size(page.getSize())
+                .totalElements(page.getTotalElements())
+                .totalPages(page.getTotalPages())
+                .first(page.isFirst())
+                .last(page.isLast())
+                .hasNext(page.hasNext())
+                .hasPrevious(page.hasPrevious())
+                .build();
+    }
+}
+

core/src/main/java/com/opencontext/config/AsyncConfig.java

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+package com.opencontext.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.scheduling.annotation.EnableAsync;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
+
+import java.util.concurrent.Executor;
+
+/**
+ * Asynchronous processing configuration.
+ * Configuration for executing heavy operations like document ingestion pipeline 
+ * in separate threads.
+ */
+@Configuration
+@EnableAsync
+public class AsyncConfig {
+
+    /**
+     * Dedicated thread pool configuration for document ingestion pipeline.
+     */
+    @Bean(name = "ingestionTaskExecutor")
+    public Executor ingestionTaskExecutor() {
+        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+        executor.setCorePoolSize(2); // Number of concurrent document ingestion tasks
+        executor.setMaxPoolSize(4);  // Maximum number of threads
+        executor.setQueueCapacity(10); // Queue capacity for waiting tasks
+        executor.setThreadNamePrefix("ingestion-");
+        executor.setRejectedExecutionHandler(new java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy());
+        executor.initialize();
+        return executor;
+    }
+
+    /**
+     * Default thread pool configuration for general asynchronous tasks.
+     */
+    @Bean(name = "taskExecutor")
+    public Executor taskExecutor() {
+        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+        executor.setCorePoolSize(4);
+        executor.setMaxPoolSize(8);
+        executor.setQueueCapacity(20);
+        executor.setThreadNamePrefix("async-");
+        executor.setRejectedExecutionHandler(new java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy());
+        executor.initialize();
+        return executor;
+    }
+}
+

core/src/main/java/com/opencontext/config/JpaAuditingConfig.java

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+package com.opencontext.config;
+
+import org.springframework.context.annotation.Configuration;
+import org.springframework.data.jpa.repository.config.EnableJpaAuditing;
+
+/**
+ * JPA Auditing configuration.
+ * Enables automatic field updates for @CreatedDate, @LastModifiedDate, etc.
+ * in entities with @EntityListeners(AuditingEntityListener.class).
+ */
+@Configuration
+@EnableJpaAuditing
+public class JpaAuditingConfig {
+}

core/src/main/java/com/opencontext/config/WebConfig.java

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+package com.opencontext.config;
+
+import org.springframework.context.annotation.Configuration;
+import org.springframework.web.servlet.config.annotation.CorsRegistry;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
+
+/**
+ * Web-related configuration.
+ * Handles web-related settings such as CORS, interceptors, resource handlers, etc.
+ */
+@Configuration
+public class WebConfig implements WebMvcConfigurer {
+
+    /**
+     * CORS configuration.
+     * Allows CORS for communication with frontend in development environment.
+     */
+    @Override
+    public void addCorsMappings(CorsRegistry registry) {
+        registry.addMapping("/api/**")
+                .allowedOriginPatterns("*") // Allow all origins in development environment
+                .allowedMethods("GET", "POST", "PUT", "DELETE", "OPTIONS","PATCH")
+                .allowedHeaders("*")
+                .allowCredentials(true)
+                .maxAge(3600);
+    }
+}
+