[Bug]: webhook enqueue failure strands event as queued with no recovery (P1)

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Problem

src/github/webhook.ts:44 records the event status: queued then calls c.env.JOBS.send with no try/catch, and no sweeper requeues stuck queued rows. If the queue send rejects, the row stays queued forever and the dedup check suppresses GitHub redeliveries → silently dropped PR gate/advisory.

Fix

Wrap JOBS.send in try/catch → mark row error on failure (dedup excludes status===error, letting redelivery through); and/or a scheduled sweeper re-enqueues queued rows older than N minutes.

Acceptance

• Simulated queue-send failure leaves the row recoverable (redelivery processes it) — test covers.