Close refund system and gate raw-PAN card entry

Refund system (Round 3 fixes + follow-up + alignment):
- Serialize cancellation refunds against the manual handler via
  per-payment advisory locks taken before the prior-refunds read
  (pg_advisory_xact_lock, ascending, same crussell:refund: key space)
- Aggregate pending cancellation refunds into ONE Square refund per
  charge (stable charge-level -square-agg key); atomic group UPDATE
  keeps crash-retry amounts identical for Square key-dedup
- Persist paymentID-square-amount idempotency keys on cancellation
  refunds; scheduler reads the stored key (legacy fallback for old rows)
- Add sweep-pending-square-refunds cron (*/5, concurrency 1) with
  refund_attempts cap; sweep retries stale manual pending refunds with
  each row's own stored idempotency key
- Reconcile at Square (GET /v2/refunds ListPaymentRefunds) before every
  terminal failed transition: tri-state result leaves rows pending on
  reconcile error instead of false-failing; PAYMENT_ALREADY_REFUNDED
  resolves to completed
- Move over-refund guard inside the lock, counting completed + pending
  (excluding failed); ErrRefundDeclined distinguishes definitive vs
  ambiguous outcomes
- forgiveFees now executes a real full refund (forceFullRefund override)
  with admin_forgiven_fees reason threaded to Square
- Surface failed card refunds in the admin notification centre
  (refund_failed enum, RETURNING-id pre-pass inserts, NOT EXISTS dedup)
- Dedup double-cancel refund inserts via ON CONFLICT (idempotency_key)
  DO NOTHING without consuming refundRemaining

Frontend:
- Remove all raw-PAN card entry: zero card_number/card_cvc/new_card_token
  in request bodies; gate new-card entry behind CardEntryUnavailable
  notice + newCardDisabled prop across all 8 flows
- Delete hand-rolled CardInput.svelte; keep CardSelection saved-card UI
  and CardEntryUnavailable fallback
- Update cancellation-policy page to in-person cash pickup wording

Tests:
- Rewrite the two amount-blind dedup tests to assert real money movement
  (single call, aggregated amount, shared refund ID)
- Add coverage: manual refund vs cancellation serialization (concurrent
  goroutines), reconcile error vs no-match branches, stale manual retry,
  forgive-fees real refund row + reason, double-cancel dedup, mock refund
  key dedup, ListPaymentRefunds filtering
- Fix time-dependent booking flakes with fixtures.NextWorkingDayAt
- 25/25 packages pass; -race clean on payments/square/db/jobs/bookings

backend/testutils/fixtures/fixtures.go

@@ -9,6 +9,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"crussell/clock"
 	"crussell/db"
 	"golang.org/x/crypto/bcrypt"
 )
@@ -178,6 +179,15 @@ func CreateTestBookingAtTime(q db.Querier, userID, serviceID string, startTime t
 	return bookingID, nil
 }
 
+// NextWorkingDayAt returns the time at `hour` UTC on a day `daysAhead` days from
+// now. Hours in [8, 18] are guaranteed inside the fixture's Mon-Sun 08:00-20:00
+// London working hours at any wall-clock time (London is at most UTC+1), unlike
+// clock.Now().Add(N * time.Hour), which can land after closing and flake tests.
+func NextWorkingDayAt(daysAhead, hour int) time.Time {
+	day := clock.Now().AddDate(0, 0, daysAhead)
+	return time.Date(day.Year(), day.Month(), day.Day(), hour, 0, 0, 0, time.UTC)
+}
+
 func CreateTestVerifiedUser(q db.Querier) (string, error) {
 	return createTestUser(q, "Verified", "User", "verified@test.com", "verified_email")
 }