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/dto/search-query.dto.ts

@@ -9,6 +9,12 @@ export class SearchQueryDto {
   @IsString({ message: "q (query) must be a string" })
   q!: string;
 
+  @IsOptional()
+  @Transform(({ value }) => (typeof value === "string" ? value.split(",") : (value as string[])))
+  @IsArray({ message: "tags must be an array" })
+  @IsString({ each: true, message: "each tag must be a string" })
+  tags?: string[];
+
   @IsOptional()
   @IsEnum(EntryStatus, { message: "status must be a valid EntryStatus" })
   status?: EntryStatus;