Liberally accept literal DSN arguments to --dsn

[rolandwalker]

Description

Forgive the user for thinking that --dsn can accept a literal DSN in addition to an alias.

Internally, recast the confusing variable name dsn to dsn_alias.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

Findings

1. Missing test coverage for new --dsn literal behavior
   The PR adds support for passing a literal DSN via --dsn, but test/test_main.py only covers --dsn with aliases. Add a test that invokes --dsn mysql://... and asserts the parsed connection args (and that CLI flags override DSN fields, mirroring existing DSN-via-positional tests). This guards the new behavior from regressions.
   File: test/test_main.py

No security issues or correctness regressions jumped out in the code paths touched.

Suggested next steps

1. Add a test in test/test_main.py for --dsn with a literal DSN, including a variant where --user/--password/--host/--port/--database override fields from the DSN.