fix(automation): recover stale outbox delivering leases

_claim_due_events flipped rows to 'delivering' and committed before the
HTTP call; if the process died between that commit and the outcome-
recording transaction, the row stayed 'delivering' forever with no reclaim
path -- a real gap, not previously documented as an accepted limitation.

Give each claim a lease deadline (reusing next_attempt_at, since it's only
otherwise meaningful for pending-status backoff scheduling) and sweep
expired leases back to pending at the start of every dispatch cycle, before
claiming new work. attempts is preserved so the count still reflects true
history. Only leases past their deadline are touched, so a still-alive
worker mid-delivery is never disturbed or double-processed.

backend/app/core/config.py

@@ -22,6 +22,7 @@ class Settings(BaseSettings):
     n8n_dispatch_interval_seconds: float = 3.0
     n8n_http_timeout_seconds: float = 5.0
     n8n_max_attempts: int = 5
+    n8n_delivery_lease_seconds: float = 120.0
     app_secret: str = "replace-in-production"
     session_cookie_name: str = "mobilityops_session"
     session_ttl_seconds: int = 60 * 60 * 8