Handle Stripe webhook events.

Flow:

1. Verify signature (400 on failure).
2. Atomic idempotency check: insert into billing_events keyed by stripe_event_id. If the row already existed, treat as a replay and return 200 without running handlers.
3. Dispatch to per-event-type handler.
4. Mark event processed.

The reconciliation logic itself lives in module-level handle* helpers so unit tests can drive them directly.