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

decnet/cli.py

@@ -24,13 +24,11 @@ from decnet.env import (
 )
 from decnet.archetypes import Archetype, all_archetypes, get_archetype
 from decnet.config import (
-    DeckyConfig,
     DecnetConfig,
-    random_hostname,
 )
 from decnet.distros import all_distros, get_distro
 from decnet.fleet import all_service_names, build_deckies, build_deckies_from_ini
-from decnet.ini_loader import IniConfig, load_ini
+from decnet.ini_loader import load_ini
 from decnet.network import detect_interface, detect_subnet, allocate_ips, get_host_ip
 from decnet.services.registry import all_services