Feed the F-Droid repo from Forgejo releases automatically

The scaffold here was a manual pipeline: copy an APK into repo/, run
`fdroid update`, rsync the result somewhere. Every app already builds and
signs its own APK in CI and publishes it as a Forgejo release, so all of
that copying was re-doing by hand what the forge already had.

Both halves of a listing now come from the same release: the APK from the
release asset, and the title, summary, description, changelog, icon and
screenshots from the fastlane/ tree at the same tag. A listing can no longer
describe a build other than the one it ships, because there is no second copy
of the text to drift.

fdroid-sync.py is the whole reconcile. It is a pure function of what the forge
currently holds — work out what should be published, make it so, re-sign the
index only if something moved — which is what makes it safe to drive from a
webhook that has no delivery guarantee and no retry. A missed delivery costs
latency, not correctness.

Three things were only found by running the real thing with an empty
environment, and all three fail late and confusingly:

- fdroidserver bundles apksigner but no JDK, and shells out to jar, keytool
  and jarsigner by bare name. The PATH is now set outright rather than
  inherited, because the bundled apksigner is a Gradle-style launcher that
  also needs xargs and sed — and without xargs fdroidserver reads every APK
  as having an invalid signature and quarantines the lot into archive/.
- index-v2 only carries a changelog for a version matching the last Builds
  entry, and a repo of prebuilt APKs has no build recipes. Without a declared
  entry every release shipped an empty "What's New".
- fdroidserver does not fall back to the APK's launcher icon, so an app whose
  fastlane tree has no images/icon.png was listed with no icon at all, which
  is two of these four. The sync now recovers one from the APK.

module.nix keeps the keystore password out of the Nix store via
fdroidserver's {env: ...} indirection, and serves the repo rooted at repo/
rather than at the working directory, which would also have handed out
config.yml and metadata/.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,7 +1,15 @@
+# The repo's index signing key and the config that holds its password. The key
+# is the repo's identity to every subscriber: back it up off this machine.
+# Server-side it lives in bucur's sops store, not here.
 keystore.p12
 config.yml
+
+# Local scratch from running fdroid by hand; the real tree lives on the server.
 repo/
-unsigned/
+metadata/
 archive/
+unsigned/
 tmp/
 stats/
+result
+__pycache__/