Bundle pyenv Python with pre-built tarball for zero-compile installs

- Combined tarball includes pyenv Python 3.12 + venv with all deps
- Downloads from GitHub releases by default (~50MB)
- Reduces install time from 20+ min to ~2 min
- Add --no-prebuilt / --from-source flags to force compile
- Update BUILD_IMAGE.md with tarball creation instructions
- Rename tarball: stegasoo-pi-arm64.tar.zst (was venv-only)

Fresh Pi installs no longer need to compile Python or jpegio.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -86,5 +86,6 @@ pishrink.sh
 frontends/web/temp_files/
 rpi/config.json
 
-# Pre-built venv tarball (39MB - too large for git)
+# Pre-built Pi tarballs (too large for git)
+rpi/stegasoo-pi-arm64.tar.zst
 rpi/stegasoo-venv-pi-arm64.tar.zst