Refactored the file processing status streaming endpoint to avoid holding
a database connection for the entire stream duration (up to 2 hours).
Changes:
- Each status poll now creates its own short-lived database session instead
of capturing the request's session in the generator closure
- Increased poll interval from 0.5s to 1s, halving database queries with
negligible UX impact
This prevents a single file status stream from blocking a connection pool
slot for hours, which could contribute to pool exhaustion under load.
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