fix: correct migration 061 checksum and add missing BillingCache mock methods

- Fix fileChecksum for 061 migration: use TrimSpace hash (66207e7a) instead
  of raw sha256sum (97bdd9a3), matching the actual runtime computation
- Add 222b4a09 as accepted DB checksum for 061 migration
- Add missing GetAPIKeyRateLimit/SetAPIKeyRateLimit/UpdateAPIKeyRateLimitUsage/
  InvalidateAPIKeyRateLimit methods to mock BillingCache in test stubs
- Fix NewBillingCacheService call in singleflight test (add apiKeyRepo param)

backend/internal/repository/migrations_runner.go

@@ -67,9 +67,10 @@ var migrationChecksumCompatibilityRules = map[string]migrationChecksumCompatibil
 		},
 	},
 	"061_add_usage_log_request_type.sql": {
-		fileChecksum: "97bdd9a32d921986f74a0231ab90735567a9234fb7062f4d9d1baf108ba59769",
+		fileChecksum: "66207e7aa5dd0429c2e2c0fabdaf79783ff157fa0af2e81adff2ee03790ec65c",
 		acceptedDBChecksum: map[string]struct{}{
 			"08a248652cbab7cfde147fc6ef8cda464f2477674e20b718312faa252e0481c0": {},
+			"222b4a09c797c22e5922b6b172327c824f5463aaa8760e4f621bc5c22e2be0f3": {},
 		},
 	},
 }