feat: 拆分 providers.json 为按 Provider 独立文件

将 compute/providers.json（54KB, 14 providers, 99 models）拆分为
compute/providers/ 目录下的独立 JSON 文件，便于维护和按需更新。

新增文件：
- compute/providers/_index.json — 加载顺序索引（保证 digest 确定性）
- compute/providers/{provider}.json × 14 — 各 provider 独立配置

保留 compute/providers.json 供旧客户端向后兼容（过渡期）。

presetDataVersion: 2 → 3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compute/providers/stability.json

@@ -0,0 +1,42 @@
+{
+  "id": "provider-stability-001",
+  "provider": "stability",
+  "label": "Stability AI",
+  "baseUrl": "https://api.stability.ai/v2beta",
+  "apiFormat": "openai-completions",
+  "apiKeyRef": "stability",
+  "apiKeyVerified": false,
+  "enabled": false,
+  "status": "unconfigured",
+  "services": [
+    "image_gen"
+  ],
+  "models": [
+    {
+      "modelName": "stable-diffusion-3.5-large",
+      "displayName": "Stable Diffusion 3.5 Large",
+      "serviceType": "image_gen",
+      "description": "Stability AI 旗舰文生图模型",
+      "capabilities": [
+        "image_generation",
+        "high_quality",
+        "controllable"
+      ],
+      "extra": {
+        "supportedImageSizes": [
+          "1024x1024",
+          "1152x896",
+          "896x1152",
+          "1216x832",
+          "832x1216"
+        ],
+        "supportedStyles": [
+          "photographic",
+          "digital-art",
+          "anime",
+          "comic-book"
+        ]
+      }
+    }
+  ]
+}