Move update source to Gitea (git.beckm4nn.net)

Releases API for the version check; the APK itself is served from the
orphan 'apk' branch of the public repo (asset uploads need a token,
raw files do not). Transitional GitHub release carries this change so
existing installs hop over.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/build.gradle.kts

@@ -12,8 +12,8 @@ android {
         applicationId = "dev.castarr.tv"
         minSdk = 26
         targetSdk = 35
-        versionCode = 18
-        versionName = "0.8.2"
+        versionCode = 19
+        versionName = "0.8.3"
     }
 
     // Release signing from environment (see ~/.keys/castarr-release.env on the