fix: generate concise release notes from the actual diff

[andre487]

Release changelogs were inferred from commit messages and file statistics, and GitHub reused F-Droid's 500-character summary. The generator now receives the actual diff and produces separate English/Russian notes for GitHub and F-Droid. The baseline is the highest reachable published stable release with all five uploaded APKs and checksums, so bare tags and failed or incomplete releases do not hide changes.

Both formats use concise bullets with one sentence per change. The prompt requires concrete behavior, checks commit intent against the net diff, and names maintenance work when no app behavior changed. GitHub notes are committed separately and validated before finalization; publication uses that committed text. Historical notes remain unchanged.

Adds bundle exec fastlane android release_notes_preview version:1.1.3 ref:v1.1.3 for a read-only preview and logs the returned model, range and token usage without credentials.

Validation:

• bundle exec fastlane android python_checks: 64 tests passed, Black/isort passed (PYTHON points to a temporary environment with pinned requirements).
• Release workflow YAML parsed successfully; git diff --check passed.
• Live preview of v1.1.3 with gpt-6.1-sol automatically selected v1.1.1, skipped unpublished v1.1.2 and recovered the configuration/subscription fixes omitted by tag-only comparison; no release files or published assets were modified.
• Tests cover incomplete, missing, empty, draft and prerelease assets, reachability, current-tag exclusion and pagination.

APK artifacts

• Download debug APK — signed with the disposable Android debug key
• Download unsigned release APK — verified unsigned

Built from commit 6610904c by CI run #327.
Artifacts expire after 14 days. Neither APK uses the MegaProxy release key.