fix: prevent imports stuck in Pending on stale DB connections

[level09]

Summary

Web/upload imports could silently get stuck in Pending with no error log on the import row, even though the celery worker logged a traceback. We hit this in production when Postgres dropped an idle connection: etl_process_file raised PendingRollbackError, the original exception was masked, and the import row was never marked Failed.

Two fixes:

1. Defensive error handling in etl_process_file (enferno/tasks/data_import.py)

   • Call db.session.rollback() before reusing the session in the except branch. Otherwise a poisoned session causes the fail-marker query to raise, masking the real exception and leaving the row stuck in Pending.
   • Guard the fail-marker itself so a secondary failure logs and doesn't replace the original exception.

2. Connection health checks on the SQLAlchemy engine (enferno/settings.py)

   • pool_pre_ping=True: ping each pooled connection before use, so workers don't get handed a dropped connection.
   • pool_recycle=300 (overridable via SQLALCHEMY_POOL_RECYCLE): cycle connections every 5 min, well under typical NAT/firewall idle timeouts.

Together: the first fix makes the symptom impossible regardless of root cause; the second stops the underlying class of error from happening in the first place.

Observed in production

PendingRollbackError: Can't reconnect until invalid transaction is rolled back.
  File "enferno/tasks/data_import.py", line 33, in etl_process_file
    log = db.session.get(DataImport, data_import_id)

The line number points inside the except clause; the original exception inside MediaImport(...) / di.process(file) was lost. We also saw OperationalError: server closed the connection unexpectedly on the same worker pool earlier the same day, confirming the connection-drop pattern.

Test plan

• Run existing test suite (uv run pytest) on a Postgres with a low idle timeout and confirm no regressions.
• Trigger a web import and verify normal happy path still produces a bulletin and Ready status.
• Force a transient DB error during etl_process_file (e.g. kill the worker's connection) and confirm the import row ends in Failed with the original exception in the log instead of Pending.

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