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/test_ssh.py

@@ -112,8 +112,8 @@ def test_dockerfile_has_rsyslog():
 
 
 def test_dockerfile_runs_as_root():
-    lines = [l.strip() for l in _dockerfile_text().splitlines()]
-    user_lines = [l for l in lines if l.startswith("USER ")]
+    lines = [line.strip() for line in _dockerfile_text().splitlines()]
+    user_lines = [line for line in lines if line.startswith("USER ")]
     assert user_lines == [], f"Unexpected USER directive(s): {user_lines}"