[AZ-484] Fix multi-source tile reads: drop Dapper enum handler

Two integration-test failures uncovered after the initial commit:

1) GetTilesByRegionAsync outer ORDER BY referenced 'updated_at' but
   the inner DISTINCT ON subquery aliased it to 'UpdatedAt' (Postgres
   folds to 'updatedat'). DISTINCT ON already guarantees one row per
   (latitude, longitude, ...) so the third tiebreak was unreachable;
   removed it.

2) Dapper 2.1.35 silently bypasses SqlMapper.TypeHandler<T> for enum
   types during read deserialization (Dapper issue #259). The
   TileSourceTypeHandler worked for writes but reads fell through to
   Enum.TryParse, which cannot map 'google_maps' to GoogleMaps.

   Pivoted: TileEntity.Source is now a string (the wire value).
   TileSource enum stays as the public producer surface in
   Common.Enums; TileSourceConverter (Common.Enums) provides
   ToWireValue / FromWireValue / IsValidWireValue at the boundary.
   TileSourceTypeHandler deleted; registration removed from
   DapperEnumTypeHandlers.RegisterAll.

   tile-storage.md Inv-5 amended to document the storage choice.
   _docs/LESSONS.md L-001 records the Dapper bypass for future cycles.

Full suite passes (213 unit + integration suite incl. AZ-484
AC-1..AC-5, security SEC-01..SEC-04, AZ-356/362/357).

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

SatelliteProvider.Common/Enums/TileSourceConverter.cs

@@ -0,0 +1,36 @@
+namespace SatelliteProvider.Common.Enums;
+
+// AZ-484: contract v1.0.0 stores TileSource as a snake_case string
+// ('google_maps', 'uav'). Dapper's TypeHandler<T> for enum types is bypassed
+// during deserialization (Dapper issue #259), so we cannot rely on a Dapper
+// handler to round-trip enum<->wire string. Instead, TileEntity stores the
+// wire value as a plain string and producers/consumers convert through this
+// helper at the boundary, preserving type safety in business code while
+// avoiding the Dapper enum read bug.
+public static class TileSourceConverter
+{
+    public const string GoogleMapsWireValue = "google_maps";
+    public const string UavWireValue = "uav";
+
+    public static string ToWireValue(TileSource value) => value switch
+    {
+        TileSource.GoogleMaps => GoogleMapsWireValue,
+        TileSource.Uav => UavWireValue,
+        _ => throw new ArgumentOutOfRangeException(nameof(value), value, "Unknown TileSource"),
+    };
+
+    public static TileSource FromWireValue(string wireValue)
+    {
+        ArgumentNullException.ThrowIfNull(wireValue);
+
+        return wireValue.ToLowerInvariant() switch
+        {
+            GoogleMapsWireValue => TileSource.GoogleMaps,
+            UavWireValue => TileSource.Uav,
+            _ => throw new ArgumentException($"'{wireValue}' is not a defined member of enum TileSource", nameof(wireValue)),
+        };
+    }
+
+    public static bool IsValidWireValue(string? wireValue) =>
+        wireValue is GoogleMapsWireValue or UavWireValue;
+}