feat: check updates from self-hosted Gitea (CN-reachable)

Switch UpdateChecker source from api.github.com (flaky in CN) to the
self-hosted Gitea at git.sunlunfan.com. Releases must now be published
to Gitea too (github stays as mirror). parseRelease unchanged — Gitea
API fields are GitHub-compatible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/com/a2i/forwarder/core/UpdateChecker.kt

@@ -59,7 +59,7 @@ class UpdateChecker(private val context: Context) {
             }
             val req = Request.Builder()
                 .url(RELEASES_API)
-                .header("Accept", "application/vnd.github+json")
+                .header("Accept", "application/json")
                 .header("User-Agent", "a2i-android")
                 .get()
                 .build()
@@ -186,7 +186,8 @@ class UpdateChecker(private val context: Context) {
     }
 
     companion object {
-        const val RELEASES_API = "https://api.github.com/repos/lsxf/a2i/releases/latest"
+        // 更新源用自建 Gitea（UPDATE_SOURCE_HIDDEN，国内可达），避免 api.github.com 不稳
+        const val RELEASES_API = "https://UPDATE_SOURCE_HIDDEN/api/v1/repos/song/a2i/releases/latest"
         const val CHECK_INTERVAL_MS = 24L * 60 * 60 * 1000
     }
 }