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/entities/knowledge-entry.entity.ts

@@ -0,0 +1,46 @@
+import { EntryStatus, Visibility } from "@prisma/client";
+
+/**
+ * Knowledge Entry entity
+ * Represents a knowledge base document/page
+ */
+export interface KnowledgeEntryEntity {
+  id: string;
+  workspaceId: string;
+  slug: string;
+  title: string;
+  content: string;
+  contentHtml: string | null;
+  summary: string | null;
+  status: EntryStatus;
+  visibility: Visibility;
+  createdAt: Date;
+  updatedAt: Date;
+  createdBy: string;
+  updatedBy: string;
+}
+
+/**
+ * Extended knowledge entry with tag information
+ */
+export interface KnowledgeEntryWithTags extends KnowledgeEntryEntity {
+  tags: Array<{
+    id: string;
+    name: string;
+    slug: string;
+    color: string | null;
+  }>;
+}
+
+/**
+ * Paginated list response
+ */
+export interface PaginatedEntries {
+  data: KnowledgeEntryWithTags[];
+  pagination: {
+    page: number;
+    limit: number;
+    total: number;
+    totalPages: number;
+  };
+}