feat: complete Dify client implementation (Day 19)

## Dify 瀹㈡埛绔皝瑁呭畬鎴?鉁?
### 瀹屾垚鐨勫伐浣?
1. 绫诲瀷瀹氫箟 (backend/src/clients/types.ts)
   - Dataset, Document, Retrieval 鐩稿叧绫诲瀷
   - 瀹屾暣鐨?TypeScript 绫诲瀷瀹氫箟
   - 鑷畾涔?DifyError 閿欒绫?
2. DifyClient 鏍稿績绫?(backend/src/clients/DifyClient.ts)
   - 鐭ヨ瘑搴撶鐞? createDataset, getDatasets, getDataset, deleteDataset
   - 鏂囨。绠＄悊: uploadDocumentDirectly, getDocuments, getDocument, deleteDocument
   - 鐭ヨ瘑搴撴绱? retrieveKnowledge (鏀寔璇箟鎼滅储銆乼op_k銆佺浉浼煎害闃堝€?
   - 杈呭姪鏂规硶: waitForDocumentProcessing, uploadAndProcessDocument

3. 娴嬭瘯鑴氭湰 (backend/src/scripts/test-dify-client.ts)
   - 8椤瑰畬鏁存祴璇曞叏閮ㄩ€氳繃
   - 娴嬭瘯鑰楁椂绾?5绉?   - 楠岃瘉鎵€鏈堿PI鍔熻兘姝ｅ父

### 娴嬭瘯缁撴灉

鉁?娴嬭瘯1: 鍒涘缓鐭ヨ瘑搴?鉁?娴嬭瘯2: 鑾峰彇鐭ヨ瘑搴撳垪琛?(鎵惧埌3涓?
鉁?娴嬭瘯3: 鑾峰彇鐭ヨ瘑搴撹鎯?鉁?娴嬭瘯4: 涓婁紶鏂囨。 (247 tokens)
鉁?娴嬭瘯5: 鑾峰彇鏂囨。鍒楄〃
鉁?娴嬭瘯6: 鐭ヨ瘑搴撴绱?(鐩镐技搴?.4420)
鉁?娴嬭瘯7: 鍒犻櫎鏂囨。
鉁?娴嬭瘯8: 鍒犻櫎鐭ヨ瘑搴?
### 鎶€鏈寒鐐?
- 瀹屽杽鐨勯敊璇鐞嗘満鍒?(axios 鎷︽埅鍣?
- 鏅鸿兘杞绛夊緟鏂囨。澶勭悊瀹屾垚
- FormData 鏂囦欢涓婁紶鏀寔
- 鍗曚緥妯″紡瀵煎嚭
- 鏀寔鑷畾涔夐厤缃?
### 渚濊禆鏇存柊

- form-data: ^4.0.0
- @types/form-data: ^2.5.0

### 閰嶇疆鏇存柊

- DIFY_API_KEY 鏇存柊涓烘湇鍔PI瀵嗛挜
- DIFY_API_URL=http://localhost/v1

### 鏂囨。鏇存柊

- 鏂板: docs/05-姣忔棩杩涘害/Day19-Dify瀹㈡埛绔皝瑁呭畬鎴?md
- 鏇存柊: docs/04-寮€鍙戣鍒?寮€鍙戦噷绋嬬.md (Day 19 鏍囪涓哄畬鎴?

### 涓嬩竴姝?
Day 20-22: 鐭ヨ瘑搴撶鐞嗗姛鑳?- 鏁版嵁搴撹〃璁捐 (KnowledgeBase, Document)
- 鍚庣 CRUD API
- 鍓嶇鐭ヨ瘑搴撶鐞嗛〉闈?- 鏂囨。涓婁紶缁勪欢

---
Progress: 閲岀▼纰?1 (MVP) 90% -> 鐭ヨ瘑搴撶鐞嗗紑鍙戜腑

backend/package-lock.json

@@ -12,9 +12,11 @@
         "@fastify/cors": "^11.1.0",
         "@fastify/jwt": "^10.0.0",
         "@prisma/client": "^6.17.0",
+        "@types/form-data": "^2.2.1",
         "axios": "^1.12.2",
         "dotenv": "^17.2.3",
         "fastify": "^5.6.1",
+        "form-data": "^4.0.4",
         "js-yaml": "^4.1.0",
         "prisma": "^6.17.0",
         "zod": "^4.1.12"
@@ -788,6 +790,15 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/form-data": {
+      "version": "2.2.1",
+      "resolved": "https://registry.npmmirror.com/@types/form-data/-/form-data-2.2.1.tgz",
+      "integrity": "sha512-JAMFhOaHIciYVh8fb5/83nmuO/AHwmto+Hq7a9y8FzLDcC1KCU344XDOMEmahnrTFlHjgh4L0WJFczNIX2GxnQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/js-yaml": {
       "version": "4.0.9",
       "resolved": "https://registry.npmmirror.com/@types/js-yaml/-/js-yaml-4.0.9.tgz",
@@ -799,7 +810,6 @@
       "version": "24.7.1",
       "resolved": "https://registry.npmmirror.com/@types/node/-/node-24.7.1.tgz",
       "integrity": "sha512-CmyhGZanP88uuC5GpWU9q+fI61j2SkhO3UGMUdfYRE6Bcy0ccyzn1Rqj9YAB/ZY4kOXmNf0ocah5GtphmLMP6Q==",
-      "dev": true,
       "license": "MIT",
       "dependencies": {
         "undici-types": "~7.14.0"
@@ -2737,7 +2747,6 @@
       "version": "7.14.0",
       "resolved": "https://registry.npmmirror.com/undici-types/-/undici-types-7.14.0.tgz",
       "integrity": "sha512-QQiYxHuyZ9gQUIrmPo3IA+hUl4KYk8uSA7cHrcKd/l3p1OTpZcM0Tbp9x7FAtXdAYhlasd60ncPpgu6ihG6TOA==",
-      "dev": true,
       "license": "MIT"
     },
     "node_modules/v8-compile-cache-lib": {