[B-01] Implement SourceDocument and DocumentChunk Entity Classes

[qowlgur121]

Objective

Implement JPA Entity classes that match the PostgreSQL data model defined in PRD.

Implementation Details

• Implement SourceDocument Entity class
• Implement DocumentChunk Entity class
• Create Repository interfaces
• Write Flyway migration scripts

Acceptance Criteria

• All entities match PRD schema specifications
• Tests pass in Docker environment

Related Documentation

• Reference PRD Section 5.1: PostgreSQL Data Modelaa

[qowlgur121]

Completed via merged PR #6