Add monorepo scaffolding and project documentation

Sets up the Nuvolari monorepo layout (app/, backend/, docs/, tool/) with the
groundwork that does not depend on the Flutter toolchain: gitignore covering
Flutter, Python and every secret file shape; env.example.json as the template
for --dart-define-from-file; a verification script that skips stages whose
target does not exist yet so it is runnable from day one; and a CI workflow in
GitHub Actions syntax so it runs unchanged on Gitea or GitHub.

The documentation records facts verified against the live services rather than
restated from the brief. Two of them change the design:

- DPC VMI rasters are 1200x1400 Float32 on a 1 km grid in a custom projection
  centred on Italy, not EPSG:4326 or EPSG:3857, and their GeoKeys are
  internally inconsistent. Reprojection is mandatory and the source CRS must be
  read from each file rather than hardcoded.
- The ARPA CAP feed carries six level values, not four: BIANCO for the
  avalanche scale out of season and "-" for no data. Collapsing either into
  VERDE would report "no alert" where the bulletin reports "not assessed".

Also documents why the frames we publish inherit CC BY-SA from the DPC source,
and why rain notifications subscribe to cell topics from the device so no user
location ever reaches a server.

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

Nuvolari/.gitignore

@@ -0,0 +1,64 @@
+# ---------------------------------------------------------------------------
+# Nuvolari — Flutter app + Python worker
+# The repository root (Europa/.gitignore) only covers Visual Studio artifacts.
+# ---------------------------------------------------------------------------
+
+# Secrets — never commit. Use env.example.json as the template and pass the
+# real file with `flutter run --dart-define-from-file=env.json`.
+env.json
+.env
+.env.*
+!.env.example
+key.properties
+*.jks
+*.keystore
+google-services.json
+GoogleService-Info.plist
+service-account*.json
+
+# Dart / Flutter
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+build/
+**/doc/api/
+*.iml
+migrate_working_dir/
+coverage/
+
+# Android
+app/android/local.properties
+app/android/.gradle/
+app/android/captures/
+app/android/key.properties
+*.apk
+*.aab
+
+# iOS / macOS
+app/ios/Pods/
+app/ios/.symlinks/
+app/ios/Flutter/Flutter.framework
+app/ios/Flutter/Flutter.podspec
+app/ios/Flutter/Generated.xcconfig
+app/ios/Flutter/flutter_export_environment.sh
+**/.DS_Store
+
+# Python worker
+.venv/
+venv/
+__pycache__/
+*.py[cod]
+.pytest_cache/
+.ruff_cache/
+.mypy_cache/
+*.egg-info/
+
+# Worker output — generated radar frames and manifests, never versioned
+backend/out/
+backend/.cache/
+
+# Editors
+.idea/