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_http_live.py

@@ -28,8 +28,8 @@ class TestHTTPLive:
         )
         lines = drain()
         # body field present in log line
-        assert any("body=" in l for l in lines if "request" in l), (
-            f"Expected 'body=' in request log line. Got:\n" + "\n".join(lines[:10])
+        assert any("body=" in line for line in lines if "request" in line), (
+            "Expected 'body=' in request log line. Got:\n" + "\n".join(lines[:10])
         )
 
     def test_method_and_path_in_log(self, live_service):