[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>

_docs/02_tasks/_dependencies_table.md

@@ -37,7 +37,7 @@ Roadmap: `_docs/04_refactoring/03-code-quality-refactoring/analysis/refactoring_
 | AZ-354 | C04 | Strict CORS by default | 1 | — | 2 | Done (In Testing) |
 | AZ-353 | C03 | Sanitize 5xx responses via IExceptionHandler | 1 | — | 3 | Done (In Testing) |
 | AZ-359 | C07 | Consolidate RegionService catch ladder | 2 | — | 3 | Done (In Testing) |
-| AZ-357 | C06 | Drop tile Version concept; new migration | 2 | — | 5 | To Do |
+| AZ-357 | C06 | Drop tile Version concept; new migration | 2 | — | 5 | Done (In Testing) |
 | AZ-362 | C09 | Idempotent POST contract | 2 | AZ-353 | 3 | To Do |
 | AZ-366 | C13 | Consolidate Haversine + filename parser | 3 | — | 2 | To Do |
 | AZ-377 | C24 | Consolidate Earth constants + 111000 | 3 | AZ-371 | 2 | To Do |