Add Gmail thread ID to queries, harden cache validation, and clean up API layer

[wesm]

Summary

This started as adding Gmail thread IDs to MCP responses but grew to include cache validation hardening and API cleanup that came up along the way.

1. Gmail Thread ID (source_conversation_id) in query layer

• Add SourceConversationID field to MessageSummary and MessageDetail models
• DuckDB engine joins conversations parquet table; SQLite engine joins conversations table
• Export conversations table to Parquet in build-cache (including Windows CSV fallback path)
• MCP tools (list_messages, get_message, search_messages) now return source_conversation_id

2. Cache validation hardening

• Shared RequiredParquetDirs list used by cache builder, TUI, and MCP startup
• HasCompleteParquetData() checks all required tables exist (not just messages)
• TUI and MCP use HasCompleteParquetData instead of HasParquetData — prevents DuckDB errors when tables like conversations are missing
• missingRequiredParquet() detects partial/broken caches and triggers full rebuild (handles hive-partitioned messages layout)
• buildCacheMu mutex prevents concurrent cache writes from parallel syncs
• maxID > 0 gate avoids infinite rebuild loops for zero-message accounts
• +482 lines of test coverage for cache edge cases

3. Remove dead config field and eliminate DTO duplication

• Remove unused server.mcp_enabled config field (MCP is always available via msgvault mcp)
• Replace duplicate APIMessage, APIAttachment, StoreStats, AccountStatus structs in internal/api with type aliases to store/scheduler types
• Collapse ~80 lines of field-by-field adapter boilerplate in serve.go to direct pass-throughs
• Add backward-compat test for old configs that still contain mcp_enabled

Test plan

• All existing tests pass
• New cache tests cover: backfill missing conversations, backfill missing messages, zero-message skip, incremental no-duplicates, hive-partitioned detection
• MCP tests verify source_conversation_id in search/get/list responses
• Config backward-compat test for deprecated mcp_enabled

Originally PR #76 (from @robelkin), squashed and rebased cleanly.

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