fix(server): treat database timeout errors as client-retryable (503)

[devin-ai-integration]

closes #22314

When many deployments poll the server simultaneously, the connection pool can be exhausted and asyncpg.connect() times out. Previously these TimeoutError exceptions were unrecognized by is_client_retryable_exception(), returning HTTP 500. Clients don't retry 500s, so the user sees noisy error logs and failed requests during transient load spikes.

This PR adds three exception types to the retryable set so the server returns 503 instead, enabling automatic client retries with backoff:

 is_client_retryable_exception():
+  sqlalchemy.exc.TimeoutError      # pool checkout timeout (all pool_size + max_overflow slots busy)
+  asyncpg.TooManyConnectionsError  # PostgreSQL max_connections reached
+  TimeoutError                     # asyncpg connection creation timeout

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

Link to Devin session: https://app.devin.ai/sessions/68eca37c30704d60a68b1794aadf9a4c

[devin-ai-integration]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing devin/1781660358-fix-db-timeout-retryable (c0e2920) with main (7107daf)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩