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

k8s/server.yaml

@@ -29,8 +29,8 @@ spec:
           type: RuntimeDefault
       containers:
         - name: server
-          # 2.5.0, pinned by digest so a re-pushed tag cannot change what runs.
-          image: git.stinnesbeck.com/nils/mg4-server@sha256:93b82b24a649c7b8b6fb7c0d4ef0bfda5694689808991a211e22ddeb257a39e2
+          # 2.6.0, pinned by digest so a re-pushed tag cannot change what runs.
+          image: git.stinnesbeck.com/nils/mg4-server@sha256:35ff5973836620c1e69f06b00163cf95183f938cb9830355452d4161c6c46e33
           ports:
             - name: http
               containerPort: 8080
@@ -44,12 +44,18 @@ spec:
             # Where a phone reaches the server; the pairing QR code points here.
             - name: PUBLIC_URL
               value: https://car.k8s.stinnesbeck.com
+            # App updates: the releases of this repository, relayed to the car. Read with
+            # FORGE_TOKEN from the mg4-server Secret; leave that empty to switch updates off.
+            - name: FORGE_URL
+              value: https://git.stinnesbeck.com
+            - name: FORGE_REPO
+              value: nils/mg4
             - name: INFLUX_TOKEN
               valueFrom:
                 secretKeyRef:
                   name: influxdb-auth
                   key: admin-token
-          # INGEST_TOKEN, APPROVAL_PASSWORD.
+          # INGEST_TOKEN, APPROVAL_PASSWORD, FORGE_TOKEN.
           envFrom:
             - secretRef:
                 name: mg4-server