[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/components/03_tile_downloader/description.md

@@ -0,0 +1,74 @@
+# TileDownloader
+
+## 1. High-Level Overview
+
+**Purpose**: Acquires satellite imagery tiles from Google Maps, stores them on disk, and persists metadata to the database. Handles session tokens, concurrent downloads, retry logic, and tile deduplication.
+
+**Architectural Pattern**: Service + Gateway (wraps external API with retry/throttling)
+
+**Upstream dependencies**: Common (DTOs, GeoUtils, configs), DataAccess (TileEntity, ITileRepository)
+
+**Downstream consumers**: RegionProcessing (via ITileService), WebApi (GoogleMapsDownloaderV2 directly for single-tile endpoints)
+
+## 2. Internal Interfaces
+
+### Class: GoogleMapsDownloaderV2
+| Method | Input | Output | Async | Error Types |
+|--------|-------|--------|-------|-------------|
+| `DownloadSingleTileAsync` | lat, lon, zoomLevel, CancellationToken | `DownloadedTileInfoV2` | Yes | ArgumentException, RateLimitException, HttpRequestException |
+| `GetTilesWithMetadataAsync` | center, radiusM, zoom, existingTiles, CancellationToken | `List<DownloadedTileInfoV2>` | Yes | ArgumentException, RateLimitException, HttpRequestException |
+
+### Service: TileService (implements ITileService)
+| Method | Input | Output | Async | Error Types |
+|--------|-------|--------|-------|-------------|
+| `DownloadAndStoreTilesAsync` | lat, lon, sizeM, zoom, CancellationToken | `List<TileMetadata>` | Yes | propagated from downloader |
+| `GetTileAsync` | Guid | `TileMetadata?` | Yes | NpgsqlException |
+| `GetTilesByRegionAsync` | lat, lon, sizeM, zoom | `IEnumerable<TileMetadata>` | Yes | NpgsqlException |
+
+## 4. Data Access Patterns
+
+### Caching Strategy
+| Data | Cache Type | TTL | Invalidation |
+|------|-----------|-----|-------------|
+| Tile bytes | In-memory (IMemoryCache, WebApi layer) | 1h absolute, 30min sliding | None (manual restart) |
+| Tile metadata | Database | Until year rollover | Version-based (current year) |
+| Active downloads | ConcurrentDictionary | Duration of download | Removed on completion |
+
+## 5. Implementation Details
+
+**Algorithmic Complexity**: Tile grid calculation is O(w×h) where w×h is the number of tiles covering the bounding box.
+
+**State Management**: `_activeDownloads` (ConcurrentDictionary) prevents duplicate concurrent downloads. `_downloadSemaphore` limits parallelism.
+
+**Key Dependencies**:
+| Library | Version | Purpose |
+|---------|---------|---------|
+| Newtonsoft.Json | 13.0.4 | Serialize session creation request body |
+| IHttpClientFactory | built-in | Create HttpClient instances per request |
+
+**Error Handling**:
+- Exponential backoff retry for 429 (rate limit) and 5xx errors: 1s → 2s → 4s → 8s → 16s, max 30s, 5 retries
+- Immediate throw for 401/403 (auth errors) and cancellation
+- `RateLimitException` thrown after exhausting retries on 429
+
+## 7. Caveats & Edge Cases
+
+- `GoogleMapsDownloaderV2` is registered as a concrete singleton (not behind an interface), creating tight coupling in `TileService` and `Program.cs`
+- User-Agent header spoofs Chrome — could be rejected if Google changes detection
+- Allowed zoom levels hardcoded to [15,16,17,18,19] — throws for others
+- Session token rotation threshold (100 tiles) is an educated guess; Google's actual limit is not documented
+- Static `_activeDownloads` dictionary means deduplication is process-wide, surviving service scope boundaries
+
+## 8. Dependency Graph
+
+**Must be implemented after**: Common, DataAccess
+**Can be implemented in parallel with**: nothing (needs both foundations)
+**Blocks**: RegionProcessing
+
+## 9. Logging Strategy
+
+| Log Level | When | Example |
+|-----------|------|---------|
+| ERROR | Download failure, session token failure | `Tile download failed. Tile: (X, Y), Status: {StatusCode}` |
+| WARN | Rate limiting retry | `Rate limited (429). Waiting {Delay}s before retry` |
+| INFO | — | (no INFO-level logs in this component) |