An exact duplicate active waitlist entry (same client/pet/service/
preferred date+time) violates the partial unique index
idx_waitlist_active_unique, which postgres-js surfaces as SQLSTATE
23505. The POST /portal/waitlist handler did not catch it, so the
duplicate returned a generic 500. Catch 23505 specifically and return
409 with a friendly message; unrelated errors still surface as 500 and
the first insert still returns 201.
Adds integration coverage: 201 first insert, 409 duplicate, 500 unrelated.
Flea Flicker