[#1142] Make webhook delivery durable and replay-safe

[arisu6804]

Summary

• add durable tenant/event-keyed delivery records with payload fingerprints
• add atomic claim, lease renewal, completion, retry backoff, and dead-letter transitions
• prevent duplicate and conflicting enqueues across worker restarts
• sanitize terminal errors and expose lifecycle statistics
• cover duplicate, timeout, restart, concurrency, malformed payload, and lease cases

Acceptance criteria

• one event key has at most one successful delivery
• concurrent workers cannot claim the same active lease
• restart recovery reclaims expired processing work
• duplicate, timeout, restart, concurrency, and malformed payload paths are tested

Validation

• git diff --check
• focused Vitest execution is unavailable because dependencies cannot be installed within the available disk space

Closes #1142