App 2.16.0, server 2.6.0: over-the-air updates from our own repository

The car updates itself again, from this fork's own releases, while the
repository stays private:

- publish-car (flake) creates the pre-release car-<version> on the
  forge and attaches the APK and its .sha256. It needs a forge token
  that may write releases, from ~/.config/nilsmg4/forge-token or
  $FORGE_TOKEN; it is sent in a header and never printed.
- The server relays releases and attachments (/v1/app/releases,
  /v1/app/assets/...), read with a read-only FORGE_TOKEN from its
  Secret, GitHub-shaped with download URLs back at itself. Redirects
  away from the forge are not followed; off (404) without a token.
- The app asks its server with its own paired token, allows exactly
  the server's host, downloads the APK itself instead of through the
  DownloadManager (so the token stays in the app), verifies SHA-256 and
  package name, and tries a PackageInstaller session (it may install
  silently as the system user) before falling back to Downloads.

OTA.md is rewritten for this flow; the k8s and server READMEs explain
the new token.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/build.gradle

@@ -34,8 +34,8 @@ android {
         applicationId "com.stinnesbeck.nilsmg4"
         minSdk 27
         targetSdk 36
-        versionCode 83
-        versionName "2.15.0"
+        versionCode 84
+        versionName "2.16.0"
 
         // Nothing about the server ships in the APK: its URL and ingest token are entered in
         // the app's Server tab (ServerConfig), and the database is only reachable by the server.