feat: accept payment notifications, once, from a verified sender

There was no inbound payment path at all: a button called a fake synchronously
and wrote an order. A real provider does the opposite — it charges, then tells
us, repeatedly, out of order, and sometimes long afterwards.

POST /api/payments/webhook verifies the signature before the body is parsed, so
an unsigned or tampered delivery is refused and recorded without touching an
order. Verified deliveries are stored under the provider's own event id with a
unique constraint, and applied inside the same transaction that marks them
processed: a repeat is a no-op, a crash is retried rather than half-applied.

An approval whose amount disagrees with the reviewed quote does not become an
order. Underpayment would ship artwork nobody paid for, and overpayment means
something a person should look at.

Order creation moved to app/payments.py so the webhook and the local development
checkout share one implementation and cannot drift. That also closes 3.5: the
charge happens inside the transaction that persists the order, rather than
before it.

The adapter contract is create/verify/parse. FakePayment implements it with a
real HMAC scheme so the whole path is exercised now, by tests/payment_test.py:
unsigned, tampered, underpaid, duplicate, re-sent, unknown reference, and
non-approved statuses. Connecting Mercado Pago is one adapter; no service code
changes.

PAYMENT_WEBHOOK_SECRET is optional in production on purpose. Required would
break the next Portainer render, and a guessable default would be worse than
either: with no secret configured the adapter verifies nothing and therefore
accepts nothing, which is the right state until a provider is connected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/schema.sql

@@ -64,6 +64,13 @@ CREATE TABLE IF NOT EXISTS dtf_local.operators (
  password_hash text NOT NULL, active boolean NOT NULL DEFAULT true,
  created_at timestamptz NOT NULL DEFAULT now(), last_login_at timestamptz
 );
+CREATE TABLE IF NOT EXISTS dtf_local.payment_events (
+ id uuid PRIMARY KEY, provider text NOT NULL, event_id text NOT NULL,
+ reference text, status text NOT NULL, amount_cents bigint,
+ payload jsonb NOT NULL, received_at timestamptz NOT NULL DEFAULT now(),
+ processed_at timestamptz, outcome text,
+ UNIQUE(provider, event_id)
+);
 CREATE TABLE IF NOT EXISTS dtf_local.security_events (
  id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  event text NOT NULL, details jsonb NOT NULL, created_at timestamptz NOT NULL DEFAULT now()
@@ -118,3 +125,9 @@ CREATE INDEX IF NOT EXISTS operator_sessions_username ON dtf_local.operator_sess
 
 -- security_status reads recent events; the worker prunes old ones by age.
 CREATE INDEX IF NOT EXISTS security_events_created ON dtf_local.security_events(created_at);
+
+-- The webhook looks an event up by provider and id on every delivery, and the
+-- unique constraint already indexes that pair. Only the unprocessed sweep needs
+-- its own index, and it stays the size of the backlog.
+CREATE INDEX IF NOT EXISTS payment_events_unprocessed ON dtf_local.payment_events(received_at)
+ WHERE processed_at IS NULL;