feat: set up signed release pipeline for Forgejo

Adds env-var-driven release signing, fastlane metadata for F-Droid/Obtainium
listings, and a tag-triggered .forgejo workflow that builds, signs, and
publishes the APK as a Forgejo release.

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

fastlane/metadata/android/en-US/changelogs/2.txt

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+Built-in Markdown editor, keyboard shortcuts, and D-pad navigation for phones.

fastlane/metadata/android/en-US/full_description.txt

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+A fast and minimalist note-taking app inspired by Notational Velocity and Terminal Velocity. Notes are plain Markdown files in a folder you choose, so you can sync or edit them with other tools.
+
+- One search bar to filter notes by title, content, and #tags, +projects, or @contexts, and to create a note by pressing Enter.
+- Notes open read-only with Markdown highlighting, and jump to the search match. Switch to editing with one tap.
+- Editing helps with Markdown: wrapping selections, continuing lists, and keeping indentation.
+- Keyboard shortcuts on desktop and D-pad navigation on phones.
+
+The in-app Help page describes every feature.

fastlane/metadata/android/en-US/short_description.txt

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+A fast, minimalist note-taking app inspired by Notational Velocity.

fastlane/metadata/android/en-US/title.txt

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+Muzzle Velocity