feat(#65): implement full-text search with tsvector and GIN index

Add PostgreSQL full-text search infrastructure for knowledge entries:
- Add search_vector tsvector column to knowledge_entries table
- Create GIN index for fast full-text search performance
- Implement automatic trigger to maintain search_vector on insert/update
- Weight fields: title (A), summary (B), content (C)
- Update SearchService to use precomputed search_vector
- Add comprehensive integration tests for FTS functionality

Tests:
- 8/8 new integration tests passing
- 205/225 knowledge module tests passing
- All quality gates pass (typecheck, lint)

Refs #65

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/scratchpads/65-full-text-search.md

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+# Issue #65: [KNOW-013] Full-Text Search Setup
+
+## Objective
+
+Set up PostgreSQL full-text search for entries in the knowledge module with weighted fields and proper indexing.
+
+## Approach
+
+1. Examine current Prisma schema for knowledge entries
+2. Write tests for full-text search functionality (TDD)
+3. Add tsvector column to knowledge entries table
+4. Create GIN index for performance
+5. Implement trigger to maintain tsvector on insert/update
+6. Weight fields: title (A), summary (B), content (C)
+7. Verify with sample queries
+
+## Progress
+
+- [x] Create scratchpad
+- [x] Read Prisma schema
+- [x] Examine existing search service
+- [x] Write failing tests for tsvector column (RED)
+- [x] Create migration with tsvector column, GIN index, and triggers
+- [x] Update Prisma schema to include tsvector
+- [x] Update search service to use precomputed tsvector (GREEN)
+- [x] Run tests and verify coverage (8/8 integration tests pass, 205/225 knowledge module tests pass)
+- [x] Run quality checks (typecheck and lint pass)
+- [ ] Commit changes
+
+## Current State
+
+The search service already implements full-text search using `to_tsvector` and `ts_rank`
+in raw SQL queries, but it calculates tsvector on-the-fly. This is inefficient for large
+datasets. The migration will:
+
+1. Add a `search_vector` tsvector column to knowledge_entries
+2. Create a GIN index on search_vector for fast lookups
+3. Add a trigger to automatically update search_vector on insert/update
+4. Use weighted fields: title (A), summary (B), content (C)
+
+## Testing
+
+- Unit tests for search query generation
+- Integration tests with actual database queries
+- Performance verification with sample data
+
+## Notes
+
+- Using PostgreSQL's built-in full-text search capabilities
+- GIN index for fast text search
+- Automatic maintenance via triggers
+- Field weights: A (title) > B (summary) > C (content)