[AZ-357] Refactor C06: drop tile Version concept; cumulative review batches 7-9

AZ-357 — eliminate year-based tile cache expiry (LF-1):
- Migration 012: drop 5-col unique index, dedupe by (lat,lon,zoom,
  size) keeping max(updated_at), add new 4-col unique index, make
  version column nullable + drop default. Column itself preserved
  per coderule (column drops require explicit confirmation; tracked
  in AZ-373 / C20).
- TileEntity.Version, TileMetadata.Version, DownloadTileResponse.
  Version: int -> int? (HTTP shape preserved; field still in JSON).
- TileService.DownloadAndStoreTilesAsync: drop currentVersion year
  computation and the .Where(t => t.Version == currentVersion)
  cache filter. BuildTileEntity: drop year arg; write Version=null.
- TileRepository: ON CONFLICT now 4-col; lookup queries
  ORDER BY updated_at DESC instead of version DESC.
- Tests: replace inverted BT02b with positive AZ357_AC1
  (prior-year cached tile is reused). Add BuildTileEntity_
  DoesNotPopulateVersion_AZ357 to enforce the no-write contract.
- 69 unit + 5 smoke + 3 stub-contract integration tests pass.

Cumulative code review (batches 7-9, 7 tasks): VERDICT=PASS.
Report at _docs/03_implementation/reviews/batch_09_review.md.
Zero Critical/High/Medium/Low findings. Architecture baseline
remains clean.

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

SatelliteProvider.DataAccess/Models/TileEntity.cs

@@ -12,7 +12,7 @@ public class TileEntity
     public int TileSizePixels { get; set; }
     public string ImageType { get; set; } = string.Empty;
     public string? MapsVersion { get; set; }
-    public int Version { get; set; }
+    public int? Version { get; set; }
     public string FilePath { get; set; } = string.Empty;
     public DateTime CreatedAt { get; set; }
     public DateTime UpdatedAt { get; set; }