[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.DataAccess/Models/TileEntity.cs

@@ -1,3 +1,5 @@
+using SatelliteProvider.Common.Enums;
+
 namespace SatelliteProvider.DataAccess.Models;
 
 public class TileEntity
@@ -14,6 +16,8 @@ public class TileEntity
     public string? MapsVersion { get; set; }
     public int? Version { get; set; }
     public string FilePath { get; set; } = string.Empty;
+    public TileSource Source { get; set; }
+    public DateTime CapturedAt { get; set; }
     public DateTime CreatedAt { get; set; }
     public DateTime UpdatedAt { get; set; }
 }