feat(python): support named database connections

[JaviMaligno]

Summary

• add named PostgreSQL connections to the Python implementation with legacy single-database fallback
• expose list_databases and optional database on all 14 database tools
• enforce write permissions per alias and cache one client per connection
• migrate the Python server entry point to MCP SDK v2
• run the existing PostgreSQL integrations in CI and add a live two-database routing test
• update parity documentation and package metadata

Verification

• uv run ruff format --check .
• uv run ruff check .
• uv run pytest -q (77 passed, 18 integration tests skipped without credentials)
• uv run pytest --run-integration -q against Azure PostgreSQL (95 passed)
• uv build
• TypeScript was also smoke-tested across Railway and Azure in the parent PR

Stack

This PR is intentionally based on feat/multi-database-typescript / #10. Merge #10 first, then retarget or merge this PR.

Closes #5 after the parent PR and this PR are merged.
Contributes the Python SDK v2 portion of #4.