fix: Node 26 uyumsuzluğunu çöz — node-appwrite -> appwrite SDK

Sorun:
- node-appwrite paketi 'node-fetch-native-with-agent' polyfill'i kullanıyor
- Node.js 26'nın undici implementation'ı ile uyumsuz
- 'fetch failed / InvalidArgumentError: invalid onError method' hatası
- Login dahil tüm Appwrite çağrıları başarısız

Çözüm:
- Tüm node-appwrite kullanımını browser SDK 'appwrite'a geçir
- Browser SDK native fetch kullanıyor, Node 26 uyumlu
- API key tabanlı admin client yerine session cookie tabanlı user client
- Public reads (read('any')): publicDB (auth'suz client)
- Admin CRUD: userDB(sessionSecret) (cookie'deki session)
- Storage upload doğrudan File objesi alıyor (InputFile.fromBuffer gerekmez)

Etkilenen dosyalar:
- lib/appwrite-server.ts: publicClient + sessionClient
- lib/auth.ts: requireSessionSecret eklendi
- lib/admin-actions.ts: tüm action'lar sessionClient kullanıyor
- app/actions.ts: publicDB
- lib/data.ts: publicDB
- app/admin/login/actions.ts: appwrite SDK
- app/admin/(protected)/page.tsx, medya/page.tsx: userDB/userStorage

End-to-end test edildi:
✓ Login (401 doğru hata)
✓ Public read (services)
✓ Anonim create (contact form)
✓ npm run build 23 route

package-lock.json

@@ -12,7 +12,6 @@
         "lucide-react": "^1.16.0",
         "marked": "^18.0.4",
         "next": "16.2.6",
-        "node-appwrite": "^25.0.0",
         "react": "19.2.4",
         "react-dom": "19.2.4"
       },
@@ -1599,22 +1598,6 @@
         "node": "^10 || ^12 || >=14"
       }
     },
-    "node_modules/node-appwrite": {
-      "version": "25.0.0",
-      "resolved": "https://registry.npmjs.org/node-appwrite/-/node-appwrite-25.0.0.tgz",
-      "integrity": "sha512-KpZ/3Ed8euz6r5CwjquElX3wRkNuiRuRQqjROiHK+feZ2ZX8HjjcF5IwrjTJYSNaYrmIwsZoex4L0ezzWjYWFg==",
-      "license": "BSD-3-Clause",
-      "dependencies": {
-        "json-bigint": "1.0.0",
-        "node-fetch-native-with-agent": "1.7.2"
-      }
-    },
-    "node_modules/node-fetch-native-with-agent": {
-      "version": "1.7.2",
-      "resolved": "https://registry.npmjs.org/node-fetch-native-with-agent/-/node-fetch-native-with-agent-1.7.2.tgz",
-      "integrity": "sha512-5MaOOCuJEvcckoz7/tjdx1M6OusOY6Xc5f459IaruGStWnKzlI1qpNgaAwmn4LmFYcsSlj+jBMk84wmmRxfk5g==",
-      "license": "MIT"
-    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",