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/knowledge.module.ts

@@ -0,0 +1,13 @@
+import { Module } from "@nestjs/common";
+import { PrismaModule } from "../prisma/prisma.module";
+import { AuthModule } from "../auth/auth.module";
+import { KnowledgeService } from "./knowledge.service";
+import { KnowledgeController } from "./knowledge.controller";
+
+@Module({
+  imports: [PrismaModule, AuthModule],
+  controllers: [KnowledgeController],
+  providers: [KnowledgeService],
+  exports: [KnowledgeService],
+})
+export class KnowledgeModule {}