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

templates/ftp/server.py

@@ -6,7 +6,6 @@ forwards events as JSON to LOG_TARGET if set.
 """
 
 import os
-import sys
 from pathlib import Path
 
 from twisted.internet import defer, reactor

templates/imap/server.py

@@ -18,7 +18,7 @@ NODE_NAME   = os.environ.get("NODE_NAME", "mailserver")
 SERVICE_NAME = "imap"
 LOG_TARGET  = os.environ.get("LOG_TARGET", "")
 PORT        = int(os.environ.get("PORT", "143"))
-IMAP_BANNER = os.environ.get("IMAP_BANNER", f"* OK Dovecot ready.\r\n")
+IMAP_BANNER = os.environ.get("IMAP_BANNER", "* OK Dovecot ready.\r\n")
 _RAW_USERS  = os.environ.get("IMAP_USERS", "admin:admin123,root:toor,mail:mail,user:user")
 
 VALID_USERS: dict[str, str] = {

templates/rdp/server.py

@@ -7,7 +7,6 @@ LOG_TARGET if set.
 """
 
 import os
-import sys
 
 from twisted.internet import protocol, reactor
 from twisted.python import log as twisted_log