fix cli empty-long arg panic and optional --db in memory mode

[fabracht]

Summary

• Debug builds panicked at startup before parsing on every command: passwd_data, acl_data and 15 other optional inline-content args used #[arg(long = "")], which clap registers as the empty long option --, tripping a debug_assert on the duplicate. Release builds skip the assert and silently mis-defined the flags. Each now has a distinct hidden long name (--passwd-data, --acl-data, …).
• agent start --memory-backend no longer requires --db (now required_unless_present = "memory_backend"; defaults to a temp dir the in-memory backend never touches).
• Added a Cli::command().debug_assert() regression test so a future empty/duplicate long name fails CI instead of only crashing debug binaries.

Test plan

• cargo test -p mqdb-cli cli_tests passes (cli_definition_is_valid, memory_backend_makes_db_optional)
• debug mqdb agent start --help runs without panic
• cargo make dev green