feat(knowledge): add tag management API (KNOW-003)

- Add Tag DTOs (CreateTagDto, UpdateTagDto) with validation
- Implement TagsService with CRUD operations
- Add TagsController with authenticated endpoints
- Support automatic slug generation from tag names
- Add workspace isolation for tags
- Include entry count in tag responses
- Add findOrCreateTags method for entry creation/update
- Implement comprehensive test coverage (29 tests passing)

Endpoints:
- GET /api/knowledge/tags - List workspace tags
- POST /api/knowledge/tags - Create tag
- GET /api/knowledge/tags/:slug - Get tag by slug
- PUT /api/knowledge/tags/:slug - Update tag
- DELETE /api/knowledge/tags/:slug - Delete tag
- GET /api/knowledge/tags/:slug/entries - List entries with tag

Related: KNOW-003

apps/api/src/knowledge/dto/index.ts

@@ -0,0 +1,5 @@
+export { CreateEntryDto } from "./create-entry.dto";
+export { UpdateEntryDto } from "./update-entry.dto";
+export { EntryQueryDto } from "./entry-query.dto";
+export { CreateTagDto } from "./create-tag.dto";
+export { UpdateTagDto } from "./update-tag.dto";