Support named database connections in TypeScript

[JaviMaligno]

Summary

• add POSTGRES_CONNECTIONS with named aliases, one lazy pool per alias, and a backwards-compatible default connection
• add list_databases plus an optional database argument to every existing database tool
• enforce per-alias write permissions and keep credentials out of tool schemas and responses
• document configuration and reject duplicate aliases or multiple defaults

Verification

• npm run lint
• npm run typecheck
• npm run build
• npm test: 42/42 passed against a live Railway PostgreSQL
• integration suite: 6/6 passed against a live Azure PostgreSQL Flexible Server
• cross-provider smoke: one MCP process selected distinct Railway and Azure pools by alias
• legacy SDK v1 client test remains green

Part of #5. Python parity follows in a separate PR so this reviewed TypeScript contract can be ported mechanically.

[gitguardian]

️✅ There are no secrets present in this pull request anymore.

If these secrets were true positive and are still valid, we highly recommend you to revoke them.
While these secrets were previously flagged, we no longer have a reference to the
specific commits where they were detected. Once a secret has been leaked into a git
repository, you should consider it compromised, even if it was deleted immediately.
Find here more information about risks.

---

^{_{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}}