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>

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(vitest@4.0.18(@opentelemetry/api@1.9.0)(@types/node@22.19.7)(jiti@2.6.1)(jsdom@26.1.0)(terser@5.46.0)(tsx@4.21.0)(yaml@2.8.2))
       ts-node:
         specifier: ^10.9.2
         version: 10.9.2(@swc/core@1.15.11)(@types/node@22.19.7)(typescript@5.9.3)