[AZ-484] Multi-source tile storage: source + captured_at

Add per-source tile rows to support multi-provider imagery (Google
Maps + future UAV). Migration 013 (transactional) introduces
source/captured_at columns, backfills existing rows to
(source='google_maps', captured_at=created_at), and replaces the
4-column unique index with a 5-column index that includes source.

TileRepository:
- ColumnList includes source + captured_at
- GetByTileCoordinatesAsync returns most-recent row across sources
  (ORDER BY captured_at DESC, updated_at DESC, id DESC)
- GetTilesByRegionAsync uses DISTINCT ON to pick the most-recent
  tile per cell, restoring caller-facing row order
- Insert/Update upsert on the new 5-column conflict key

TileSource enum lives in Common.Enums. Snake_case wire format
(google_maps, uav) is enforced by a focused TileSourceTypeHandler
because the generic ToLowerInvariant pattern would emit
"googlemaps", violating contract v1.0.0.

TileService stamps Source=GoogleMaps + CapturedAt=UtcNow on every
new tile. Tile-storage contract is now frozen at v1.0.0.

AC coverage 7/7. New unit + integration tests cover all ACs;
existing 200 unit + 5 smoke tests preserved.

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

SatelliteProvider.Tests/RepositoryRefactorTests.cs

@@ -125,7 +125,8 @@ public class RepositoryRefactorTests
             "latitude", "longitude",
             "tile_size_meters as TileSizeMeters", "tile_size_pixels as TileSizePixels",
             "image_type as ImageType", "maps_version as MapsVersion", "version",
-            "file_path as FilePath", "created_at as CreatedAt", "updated_at as UpdatedAt"
+            "file_path as FilePath", "source", "captured_at as CapturedAt",
+            "created_at as CreatedAt", "updated_at as UpdatedAt"
         };
         var regionColumns = new[]
         {