The remaining routes of the idle-in-transaction class: the file-manager,
storage, camera-snapshot and timelapse routes each took their printer row
via Depends(get_db) and then talked FTP/camera on the same held session, so
a farm dashboard polling cover/snapshot tiles (offline printers included)
crept the pool to exhaustion over ~23h. They now read in a short session and
release before the I/O; timelapse re-opens a fresh session only for the write.
Also caps the four bare-executor FTP helpers with asyncio.wait_for so a
saturated 48-worker pool can't pin a caller (and its DB connection)
indefinitely, and runs the synchronous smtplib send off the event loop with
an explicit timeout so a wedged relay can't freeze the loop.
