test: add tests for SQLite static data cache layer

[z4ab]

Adds 8 tests for the services/gtfs_db.py module:

Test	Scenario
test_save_and_load	Full round-trip with all fields
test_load_missing_url	Loading unsaved URL returns None
test_save_updates_existing	Upsert replaces old data
test_save_with_empty_dicts	Empty routes/stops persisted as {}
test_save_without_optional_fields	Missing optionals return {}/None
test_clear_cache_all	No-arg clear_cache deletes everything
test_clear_cache_single	URL-specific clear_cache
test_load_nonexistent_db	Non-existent DB path returns None

Closes #78