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 <[email protected]>

pnpm-lock.yaml

@@ -308,6 +308,9 @@ importers:
       '@types/node':
         specifier: ^22.13.4
         version: 22.19.7
+      '@vitest/coverage-v8':
+        specifier: ^4.0.18
+        version: 4.0.18([email protected](@opentelemetry/[email protected])(@types/[email protected])([email protected])([email protected])([email protected])([email protected])([email protected]))
       ts-node:
         specifier: ^10.9.2
         version: 10.9.2(@swc/[email protected])(@types/[email protected])([email protected])