Rename Pi tarball to stegasoo-rpi-runtime-env-arm64.tar.zst

More descriptive name for the pre-built pyenv + venv bundle.
Updated all scripts and docs to use new filename.
Also bumped PREBUILT_URL to v4.1.5.

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

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

.gitignore

@@ -87,9 +87,8 @@ frontends/web/temp_files/
 rpi/config.json
 
 # Pre-built Pi tarballs and images (release assets, too large for git)
-rpi/stegasoo-pi-arm64.tar.zst
-rpi/stegasoo-pi-arm64.tar.zst.zip
-rpi/stegasoo-venv-pi-arm64.tar.zst
+rpi/*.tar.zst
+rpi/*.tar.zst.zip
 rpi/*.img
 rpi/*.img.zst
 rpi/*.img.zst.zip