fix: resolve all ruff lint errors and SQLite UNIQUE constraint issue

Ruff fixes (20 errors → 0):
- F401: Remove unused imports (DeckyConfig, random_hostname, IniConfig,
  COMPOSE_FILE, sys, patch) across cli.py, mutator/engine.py,
  templates/ftp, templates/rdp, test_mysql.py, test_postgres.py
- F541: Remove extraneous f-prefixes on strings with no placeholders
  in templates/imap, test_ftp_live, test_http_live
- E741: Rename ambiguous variable 'l' to descriptive names (line, entry,
  part) across conftest.py, test_ftp_live, test_http_live,
  test_mongodb_live, test_pop3, test_ssh

SQLite fix:
- Change _initialize_sync() admin seeding from SELECT-then-INSERT to
  INSERT OR IGNORE, preventing IntegrityError when admin user already
  exists from a previous run

tests/live/conftest.py

@@ -83,7 +83,7 @@ def assert_rfc5424(
     criteria = {"service": service, "event_type": event_type, **fields}
     raise AssertionError(
         f"No RFC 5424 line matching {criteria!r} found among {len(lines)} lines:\n"
-        + "\n".join(f"  {l!r}" for l in lines[:20])
+        + "\n".join(f"  {line!r}" for line in lines[:20])
     )