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/update-entry.dto.ts

@@ -0,0 +1,48 @@
+import {
+  IsString,
+  IsOptional,
+  IsEnum,
+  IsArray,
+  MinLength,
+  MaxLength,
+} from "class-validator";
+import { EntryStatus, Visibility } from "@prisma/client";
+
+/**
+ * DTO for updating a knowledge entry
+ */
+export class UpdateEntryDto {
+  @IsOptional()
+  @IsString({ message: "title must be a string" })
+  @MinLength(1, { message: "title must not be empty" })
+  @MaxLength(500, { message: "title must not exceed 500 characters" })
+  title?: string;
+
+  @IsOptional()
+  @IsString({ message: "content must be a string" })
+  @MinLength(1, { message: "content must not be empty" })
+  content?: string;
+
+  @IsOptional()
+  @IsString({ message: "summary must be a string" })
+  @MaxLength(1000, { message: "summary must not exceed 1000 characters" })
+  summary?: string;
+
+  @IsOptional()
+  @IsEnum(EntryStatus, { message: "status must be a valid EntryStatus" })
+  status?: EntryStatus;
+
+  @IsOptional()
+  @IsEnum(Visibility, { message: "visibility must be a valid Visibility" })
+  visibility?: Visibility;
+
+  @IsOptional()
+  @IsArray({ message: "tags must be an array" })
+  @IsString({ each: true, message: "each tag must be a string" })
+  tags?: string[];
+
+  @IsOptional()
+  @IsString({ message: "changeNote must be a string" })
+  @MaxLength(500, { message: "changeNote must not exceed 500 characters" })
+  changeNote?: string;
+}