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>

apps/api/src/knowledge/search.controller.ts

@@ -31,6 +31,7 @@ export class SearchController {
    * Requires: Any workspace member
    *
    * @query q - The search query string (required)
+   * @query tags - Comma-separated tag slugs to filter by (optional, entries must have ALL tags)
    * @query status - Filter by entry status (optional)
    * @query page - Page number (default: 1)
    * @query limit - Results per page (default: 20, max: 100)
@@ -45,6 +46,7 @@ export class SearchController {
       status: query.status,
       page: query.page,
       limit: query.limit,
+      tags: query.tags,
     });
   }