Self-hosted APK distribution

Memby is handed out from a NAS or web server rather than a store, so:

- UpdateChecker gains a static-manifest source alongside Gitea. A .json
  update URL is read as a manifest ({version, apkUrl, notes}); anything
  else is still treated as a Gitea host. apkUrl may be relative and is
  resolved against the manifest's own URL.
- Release signing config reads memby.keystore from local.properties or
  the environment. Without it the build still succeeds but warns loudly:
  an unsigned APK will not install, and a changed key forces every user
  to uninstall before they can update.
- release.ps1 bumps the version, runs tests, builds a signed APK and
  assembles dist/out/ (landing page, latest.json, versioned APK) ready to
  copy onto the NAS.
- Keystores and dist/out are gitignored.

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

.gitignore

@@ -18,3 +18,9 @@ local.properties
 # Holds the Postgres password, admin token and Emby address.
 .env
 /server/bin/
+
+# Release artefacts and signing material. The keystore must never be committed:
+# whoever holds it can publish an update that installs over Memby.
+/dist/out/
+*.jks
+*.keystore