fix(mail): prevent concurrent setup runs

[Rajioba1]

Summary

• reserve the process-local setup slot before the first database await
• add a non-blocking Postgres advisory-lock handle for cross-replica coordination
• hold the advisory lock for the complete SSE setup lifecycle
• release the lock on completion, pause, failure, disconnect, and setup-start errors
• return 409 immediately when another replica owns the server lock
• return 503 instead of running unlocked when reservation persistence fails

Why

The previous active check and assignment were separated by an awaited database lookup, so two requests could both pass the check. The mail_servers fallback was also not an atomic lock because upsert() updates on conflict.

The former one-hour row heuristic also blocked intentional DNS/PTR resumes and could expire while a valid long-running install was still active. A session-level advisory lock follows the actual SSE lifecycle instead of relying on elapsed time.

Tests

• focused mail suite: 24 passed
• advisory-lock tests: 4 passed
• bun run --cwd apps/api lint
• bun run --cwd packages/db lint
• Prettier check on all new and directly formatted files
• independent code review: green, no remaining blockers

bun format was run per CONTRIBUTING.md. Its repository-wide Windows line-ending churn and unrelated formatting changes were excluded; the pre-existing mail.controller.ts file is not fully Prettier-clean on main.

Fixes #79