[AZ-284] Autodev baseline + testability refactor

Phase A baseline outputs from /autodev (Steps 1-5):
- Problem & solution docs (_docs/00_problem, _docs/01_solution)
- Codebase documentation (_docs/02_document) incl. architecture,
  module-layout, glossary, system-flows, baseline compliance scan
- Test specs (blackbox, performance, resilience, security, resource,
  traceability matrix)
- Test task decomposition (_docs/02_tasks/todo): AZ-285..AZ-290
- Testability refactor (_docs/04_refactoring/01-testability-refactoring):
  - TC-01 Move DownloadedTileInfoV2 + new ExistingTileInfo to Common.DTO
  - TC-02 Replace dead ISatelliteDownloader API with real signatures
  - TC-03 GoogleMapsDownloaderV2 implements ISatelliteDownloader
  - TC-04 TileService depends on ISatelliteDownloader (mockable)
  - TC-05 DI + endpoints use ISatelliteDownloader
- Test runner scripts (scripts/run-tests.sh, run-performance-tests.sh)
- Autodev state pointer (_docs/_autodev_state.md)

Prepares the codebase for AZ-285..AZ-290 unit/integration test work.

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/02_document/modules/services_google_maps_downloader.md

@@ -0,0 +1,65 @@
+# Module: Services/GoogleMapsDownloaderV2
+
+## Purpose
+Downloads satellite imagery tiles from Google Maps. Handles session token management, concurrent download throttling, retry logic with exponential backoff, and tile deduplication.
+
+## Public Interface
+
+### GoogleMapsDownloaderV2
+- Constructor: `GoogleMapsDownloaderV2(ILogger, IOptions<MapConfig>, IOptions<StorageConfig>, IOptions<ProcessingConfig>, IHttpClientFactory)`
+- `DownloadSingleTileAsync(double lat, double lon, int zoomLevel, CancellationToken) → Task<DownloadedTileInfoV2>`: downloads one tile at specified coordinates. Validates zoom level, creates session token, downloads image, saves to disk.
+- `GetTilesWithMetadataAsync(GeoPoint center, double radiusM, int zoomLevel, IEnumerable<TileEntity> existingTiles, CancellationToken) → Task<List<DownloadedTileInfoV2>>`: downloads all tiles in a bounding box, skipping those already present in `existingTiles`. Manages session token rotation.
+
+### DownloadedTileInfoV2 (record)
+- `X`, `Y` (int), `ZoomLevel` (int), `CenterLatitude`, `CenterLongitude` (double), `FilePath` (string), `TileSizeMeters` (double)
+
+### RateLimitException (exception)
+Custom exception thrown when Google Maps returns 429 Too Many Requests and retries are exhausted.
+
+## Internal Logic
+- **Allowed zoom levels**: 15, 16, 17, 18, 19 — throws `ArgumentException` for others
+- **URL template**: `https://mt{server}.google.com/vt/lyrs=s&x={x}&y={y}&z={z}&token={token}`
+- **Session tokens**: obtained via `https://tile.googleapis.com/v1/createSession?key={apiKey}`, rotated every `SessionTokenReuseCount` tiles (default: 100)
+- **Concurrency control**: `SemaphoreSlim` limits parallel downloads to `MaxConcurrentDownloads` (default: 4)
+- **Deduplication**: `ConcurrentDictionary<string, Task<DownloadedTileInfoV2>>` (`_activeDownloads`) prevents duplicate concurrent downloads of the same tile
+- **Retry logic**: exponential backoff (1s base, 30s max, 5 retries) for 429 and 5xx errors. Cancellation and auth errors (401, 403) propagate immediately.
+- **Server selection**: `(x + y) % 4` distributes requests across `mt0`–`mt3`; single-tile downloads always use `mt0`
+- **Delay between requests**: configurable via `ProcessingConfig.DelayBetweenRequestsMs`
+- **Tile size calculation**: `CalculateTileSizeInMeters` uses Earth circumference × cos(latitude) / (2^zoom × 256)
+
+## Dependencies
+- `SatelliteProvider.Common.Configs` — MapConfig, StorageConfig, ProcessingConfig
+- `SatelliteProvider.Common.DTO` — GeoPoint
+- `SatelliteProvider.Common.Utils` — GeoUtils
+- `SatelliteProvider.DataAccess.Models` — TileEntity (for existingTiles parameter)
+- NuGet: `Newtonsoft.Json`, `Microsoft.Extensions.Http`, `Microsoft.Extensions.Options`
+
+## Consumers
+- `TileService` — `GetTilesWithMetadataAsync`
+- `Program.cs` (ServeTile, GetTileByLatLon) — `DownloadSingleTileAsync`
+
+## Data Models
+Produces `DownloadedTileInfoV2` records; accepts `TileEntity` for cache checks.
+
+## Configuration
+| Config | Key | Used For |
+|--------|-----|----------|
+| MapConfig | ApiKey | Session token requests |
+| StorageConfig | TilesDirectory | File save paths |
+| ProcessingConfig | MaxConcurrentDownloads | SemaphoreSlim capacity |
+| ProcessingConfig | DelayBetweenRequestsMs | Throttle delay |
+| ProcessingConfig | SessionTokenReuseCount | Token rotation threshold |
+
+## External Integrations
+| Integration | Protocol | Details |
+|-------------|----------|---------|
+| Google Maps Tile API | HTTPS | `mt*.google.com/vt/lyrs=s` for tiles |
+| Google Maps Session API | HTTPS | `tile.googleapis.com/v1/createSession` |
+| File system | Local FS | Writes JPEG tiles to `StorageConfig.TilesDirectory` |
+
+## Security
+- API key transmitted over HTTPS to Google endpoints
+- User-Agent spoofs a Chrome browser to match expected Google Maps client
+
+## Tests
+No dedicated unit tests (the test file `GoogleMapsDownloaderTests.cs` contains only a dummy test).