The client's in-flight flag covers the menus of one page only. A push reads YouTube, then writes
both sides for seconds and commits step by step, so a row lock ends at its first commit: two tabs,
or a push and the scheduled mirror refresh, could interleave into a doubled YouTube item or a
baseline recorded over the other's writes. `sync_claim` takes a session-level advisory lock on a
connection of its own (it survives those commits; a crashed worker's lock dies with its
connection, so no claim column is left stuck). Push and revert answer 409 `sync_busy`; the
scheduled refresh skips a list being synced and picks it up next run.
peter
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