feat(#66): implement tag filtering in search API endpoint

Add support for filtering search results by tags in the main search endpoint.

Changes:
- Add tags parameter to SearchQueryDto (comma-separated tag slugs)
- Implement tag filtering in SearchService.search() method
- Update SQL query to join with knowledge_entry_tags when tags provided
- Entries must have ALL specified tags (AND logic)
- Add tests for tag filtering (2 controller tests, 2 service tests)
- Update endpoint documentation
- Fix non-null assertion linting error

The search endpoint now supports:
- Full-text search with ranking (ts_rank)
- Snippet generation with highlighting (ts_headline)
- Status filtering
- Tag filtering (new)
- Pagination

Example: GET /api/knowledge/search?q=api&tags=documentation,tutorial

All tests pass (25 total), type checking passes, linting passes.

Fixes #66

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

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

@@ -25,7 +25,7 @@ Set up PostgreSQL full-text search for entries in the knowledge module with weig
 - [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
+- [x] Commit changes (commit 24d59e7)
 
 ## Current State