Move database-dependent sync tests from internal/sync/ to
cmd/server/tests/ where they belong:
- TestSyncIntegration_OfflineDetector_* tests
- TestSyncIntegration_QueueProcessor_EnqueueProgress test
- Helper functions: setupSyncTestDB, createSyncTestUser/Device
These tests require a running PostgreSQL database and are
properly categorized as integration tests now.
Unit tests that remain in internal/sync/:
- TestOfflineDetector_ConstantValues (no DB needed)
- TestCalculateNextRetry (logic only)
- TestSyncTypeConstants, TestSyncStatusConstants (constants)
- TestPriorityConstants (constants)
All unit tests now pass with `make test` (no DB required).
