[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/TypeHandlers/TileSourceTypeHandler.cs

@@ -0,0 +1,49 @@
+using System.Data;
+using Dapper;
+using SatelliteProvider.Common.Enums;
+
+namespace SatelliteProvider.DataAccess.TypeHandlers;
+
+// AZ-484: TileSource needs an explicit string mapping because the multi-word
+// member 'GoogleMaps' must round-trip as the snake_case wire value 'google_maps'
+// per the v1.0.0 tile-storage contract. The generic EnumStringTypeHandler<T>
+// only does ToString().ToLowerInvariant(), which would emit 'googlemaps'.
+public class TileSourceTypeHandler : SqlMapper.TypeHandler<TileSource>
+{
+    public const string GoogleMapsWireValue = "google_maps";
+    public const string UavWireValue = "uav";
+
+    public override TileSource Parse(object value)
+    {
+        if (value is null || value is DBNull)
+        {
+            throw new DataException("Cannot parse null DB value into enum TileSource");
+        }
+
+        var s = value as string ?? value.ToString();
+        if (string.IsNullOrEmpty(s))
+        {
+            throw new DataException("Cannot parse empty DB value into enum TileSource");
+        }
+
+        return s.ToLowerInvariant() switch
+        {
+            GoogleMapsWireValue => TileSource.GoogleMaps,
+            UavWireValue => TileSource.Uav,
+            _ => throw new DataException($"DB value '{s}' is not a defined member of enum TileSource"),
+        };
+    }
+
+    public override void SetValue(IDbDataParameter parameter, TileSource value)
+    {
+        parameter.Value = ToWireValue(value);
+        parameter.DbType = DbType.String;
+    }
+
+    public static string ToWireValue(TileSource value) => value switch
+    {
+        TileSource.GoogleMaps => GoogleMapsWireValue,
+        TileSource.Uav => UavWireValue,
+        _ => throw new ArgumentOutOfRangeException(nameof(value), value, "Unknown TileSource"),
+    };
+}