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/app.module.ts

@@ -12,6 +12,7 @@ import { DomainsModule } from "./domains/domains.module";
 import { IdeasModule } from "./ideas/ideas.module";
 import { WidgetsModule } from "./widgets/widgets.module";
 import { LayoutsModule } from "./layouts/layouts.module";
+import { KnowledgeModule } from "./knowledge/knowledge.module";
 
 @Module({
   imports: [
@@ -26,6 +27,7 @@ import { LayoutsModule } from "./layouts/layouts.module";
     IdeasModule,
     WidgetsModule,
     LayoutsModule,
+    KnowledgeModule,
   ],
   controllers: [AppController],
   providers: [AppService],