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/test_ftp_live.py

@@ -35,5 +35,5 @@ class TestFTPLive:
         ftp.close()
         lines = drain()
         # At least one RFC 5424 line from the ftp service
-        rfc_lines = [l for l in lines if "<" in l and ">1 " in l and "ftp" in l]
-        assert rfc_lines, f"No ftp RFC 5424 lines found. stdout:\n" + "\n".join(lines[:15])
+        rfc_lines = [line for line in lines if "<" in line and ">1 " in line and "ftp" in line]
+        assert rfc_lines, "No ftp RFC 5424 lines found. stdout:\n" + "\n".join(lines[:15])