The webhook routes inbound messages by phone_number_id and uses
`.single()` to find the owning config. If two users save the same
phone_number_id, `.single()` errors PGRST116 ("multiple rows") and every
inbound message is silently dropped while the log misleadingly reports
"No config found".
Three layers of fix:
- POST /api/whatsapp/config now rejects with 409 when another user has
already claimed the phone_number_id. The check uses a service-role
client because RLS hides the conflicting row from the saving user.
- Webhook lookup no longer uses `.single()`. It distinguishes 0 rows
(true "no config") from ≥2 rows (duplicate claim) so logs name the
real cause and list conflicting user_ids.
- Migration 013 adds UNIQUE(phone_number_id) to whatsapp_config so the
same can't happen via direct DB writes or a race. Fails loudly with a
copy-pasteable resolution hint if duplicates already exist, rather
than auto-deduping (which would destroy tokens).
Closes#136.
