ci: fail the build when a changelog will not fit F-Droid

`sync_changelog_to_fastlane.sh` has been printing "note: >500 chars — F-Droid
may truncate this changelog in-client" for three releases, and every release
since 0.2.0 has sailed past it: 1824, 696, 916 characters. A note nobody acts
on is not a check.

F-Droid truncates the in-client "What's New" box, so everything past the limit
is written for nobody — the reader gets a sentence cut mid-word and no way to
expand it. The limit is now a hard failure: the script exits non-zero, with a
message naming the section to shorten, and `MAX_CHARS` is a variable so the
bound lives in one place rather than being copied into the workflows.

Enforced in two places, for two different failures:

- `.forgejo/workflows/ci.yaml` runs it as an always-on guard beside the
  reproducible-release invariant, then checks `git status --porcelain` over the
  changelogs directory. That second half catches a CHANGELOG.md edit whose
  generated fastlane file was never committed — which until now degraded
  silently into the official F-Droid listing showing the *previous* version's
  notes, exactly as RELEASING.md step 3 warns. Porcelain rather than
  `git diff --exit-code`, so a brand-new file for a bumped versionCode counts
  as dirty instead of being missed as untracked.
- `.gitea/workflows/release.yaml` gets the same call in the cheap `detect`
  gate. The release job already regenerated the file, but only at step 15 of
  24 — after the build, the signing and the keystore setup. Failing in `detect`
  costs one bash invocation and publishes nothing.

.gitea/workflows/release.yaml

@@ -110,6 +110,16 @@ jobs:
               ;;
           esac
 
+      # Before a single Gradle task runs: F-Droid truncates the in-client
+      # changelog, so an over-long one would reach users cut off mid-sentence.
+      # The script exits non-zero past the limit. Cheap enough to sit in the
+      # gate job, where failing costs nothing and publishes nothing — the step
+      # further down that regenerates the file for the repo would otherwise be
+      # the first thing to notice, after the build and the signing.
+      - name: Changelog fits F-Droid
+        if: steps.v.outputs.is_release == 'true'
+        run: bash scripts/sync_changelog_to_fastlane.sh
+
   # Releases: build + sign + publish, then mint the tag and Gitea release.
   # Also runs on manual dispatch, where it skips the build and just re-signs and
   # re-uploads the existing index (recovery path).