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/docs/stack-decisions.md

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+# Stack decisions
+
+Each entry records what was chosen, what it was chosen over, and why. Revisit an entry
+only with a reason that invalidates its rationale.
+
+## Flutter, single codebase
+
+Android ships first, iOS follows on the same code. Platform-specific work stays behind
+interfaces (`core/platform/`) so the iOS port is additive rather than a rewrite.
+
+Toolchain pinned during setup: **Flutter 3.47.3 stable / Dart 3.13.3**,
+compileSdk/targetSdk **36**, minSdk **21** (the floor imposed by `maplibre_gl`),
+JDK **21**.
+
+## State management — Riverpod without code generation
+
+`flutter_riverpod` with hand-written `Notifier` / `AsyncNotifier` classes.
+
+Chosen over: Bloc (more ceremony than this app's state needs), plain `setState`
+(the radar timeline, the frame cache and the consent flow all share state across
+screens), Riverpod *with* `riverpod_generator`.
+
+Dropping code generation keeps `build_runner` out of CI and out of every edit-run
+cycle. The generator's benefit — less boilerplate on providers — is small at this
+size, and its cost is paid on every build. Reconsider if provider count passes ~40.
+
+## Networking — Dio
+
+`dio` with interceptors for: the mandatory MET Norway User-Agent, retry with
+exponential backoff, and timeouts.
+
+Chosen over `http`, which has no interceptor model — the User-Agent obligation is a
+licensing requirement, so it belongs in one enforced place rather than at each call
+site where it can be forgotten.
+
+The DPC `origin` header lives **only in the Python worker**. The app never talks to
+DPC directly.
+
+## Models — hand-written `fromJson`
+
+Chosen over `freezed` + `json_serializable`. The model set is small (radar manifest and
+frames, forecast series, CAP alerts, region config) and the parsers are covered by
+tests against real fixtures. Same rationale as Riverpod: no `build_runner`.
+
+The tests, not the generator, are what guarantee the parsing is right — fixtures
+captured from the real endpoints catch schema drift that codegen would not.
+
+## Map — MapLibre GL
+
+`maplibre_gl` 0.27.0 (Flutter 3.29+, Android API 21+, iOS 13+).
+
+Chosen over `flutter_map`, which renders tiles in Dart. Radar animation redraws a
+full-viewport image several times a second; a GPU-composited native renderer holds
+frame rate where a Dart canvas does not.
+
+The base map style URL comes from `MAP_STYLE_URL`. With no key configured the app
+loads a minimal local style — flat background plus the Piedmont boundary from a
+bundled GeoJSON — so development, tests and the demo mode all work offline.
+
+## Radar frame rendering — image source, double buffered
+
+The worker publishes each frame already cropped to the region bounding box and
+reprojected to EPSG:3857, so the four corners of a MapLibre `LatLngQuad` are exact and
+no client-side warping is needed.
+
+Animation uses **two image sources, A and B**: while one is visible the next frame is
+decoded into the other, then visibility swaps.
+
+Chosen over: adding all ~20 frames as layers with opacity 0 (constant GPU memory
+matters more than the saved swap — 20 frames at 1024×1024 RGBA is ~80 MB resident),
+and over updating a single source in place (visible flicker during decode).
+
+## Frame cache — custom LRU
+
+`FrameCache` over `path_provider`: an on-disk LRU with a configurable byte cap, plus a
+small in-memory LRU of PNG bytes.
+
+Chosen over `flutter_cache_manager`, which does not expose the eviction control the
+scrubber needs. Prefetch must prioritise frames adjacent to the playhead and evict by
+distance from it, not by age.
+
+## Region configuration
+
+Everything region-specific lives in `app/assets/regions/<id>.json`: bounding box, map
+centre and zoom limits, alert zones, active data sources, attribution strings. Adding
+a region is a new JSON file plus its assets — no Dart changes.
+
+Piedmont bounding box, padded: `[6.55, 43.95, 9.30, 46.55]` (W, S, E, N).
+To be refined against the ISTAT geometry once that dataset is confirmed.
+
+## Backend — Python worker on a VPS
+
+GDAL and HDF5 cannot run on Supabase Edge Functions (Deno), so the worker runs as a
+scheduled process on a VPS or Cloud Run.
+
+`rasterio` 1.5.1 publishes Windows wheels for Python 3.14, so the worker also runs
+natively on the development machine — no Docker or WSL needed to iterate.
+
+Trigger: the DPC WebSocket push channel `wss://radar-wss.protezionecivile.it`, with a
+5-minute cron as fallback. Polling DPC on a timer is the fallback, never the norm.
+
+Output goes through a `Publisher` interface: `LocalPublisher` writes to `backend/out/`
+for development, `S3Publisher` targets R2 or Supabase Storage once credentials exist.
+
+## Rain notifications — client-side topic subscription
+
+The worker computes rain per **geographic cell** and publishes per-cell state. The app
+subscribes to FCM topics named after cells, **from the device**.
+
+The server therefore never learns any user's position — not precisely, not even by
+cell. There is no user table to leak, and Data safety can honestly declare that no
+location is transmitted or stored.