feat: add public-api app and route update checks through it

- Add `apps/public-api` (`@sofa/public-api`) — minimal Hono app
  deployable to Vercel; `GET /v1/version` fetches the latest GitHub
  release and returns `{ version, releaseUrl }` with a 15-minute
  CDN cache (`s-maxage=900, stale-while-revalidate=3600`)
- Update `packages/core/src/update-check.ts` to call
  `PUBLIC_API_URL/v1/version` instead of the GitHub API directly;
  `PUBLIC_API_URL` defaults to `https://public-api.sofa.watch`

bun.lock

@@ -85,6 +85,17 @@
         "typescript": "catalog:",
       },
     },
+    "apps/public-api": {
+      "name": "@sofa/public-api",
+      "version": "0.1.0",
+      "dependencies": {
+        "hono": "4.12.7",
+      },
+      "devDependencies": {
+        "@types/bun": "catalog:",
+        "typescript": "catalog:",
+      },
+    },
     "apps/server": {
       "name": "@sofa/server",
       "version": "0.1.0",
@@ -997,6 +1008,8 @@
 
     "@sofa/native": ["@sofa/native@workspace:apps/native"],
 
+    "@sofa/public-api": ["@sofa/public-api@workspace:apps/public-api"],
+
     "@sofa/server": ["@sofa/server@workspace:apps/server"],
 
     "@sofa/tmdb": ["@sofa/tmdb@workspace:packages/tmdb"],