Implement full Square payment review fixes + frontend polish

Implement every finding from the deep payment review (P0-P2, minors,
nitpicks), then close the post-implementation re-review items, then
align card-form typography and roll out the Square trust badge.

Backend - Square API alignment:
- tip_settings.allow_tipping nested under device_options (was top-level:
  terminal tips were silently lost in prod)
- CreateCardOnFile now accepts customerID and sends card.customer_id;
  saved-card (ccof:) charges forward square_customer_id as CustomerID
- New SquareClient methods GetPayment, CreateCustomer, CancelCheckout
- SCA verification_token accepted + forwarded in all charge paths
- ExpMonth/ExpYear -> *int; URL-path id validation; CancelCheckout
  NOT_FOUND-only no-op (dropped unverified NOOP); exported ErrorCode/
  ErrorDetail helpers; mock rejects raw PANs, RList locks, redacts
  emails, ForceRefundPending hook

Backend - money safety:
- sweepManualPendingSquareRefunds reconciles rows WITH square_refund_id
  instead of stranding them forever
- SweepStalePendingPayments reconciles at Square before failing (tri-state:
  leave pending on transport error, rescue completed, fail definitively)
- GetCheckoutStatus cancellation-recheck; terminal CANCELED resolution;
  SweepStaleTerminalCheckouts covers terminal_checkouts table
- till gift-card clawback on definitive failure incl. retry path +
  INSUFFICIENT_FUNDS/ADDRESS_VERIFICATION_FAILURE/TRANSACTION_LIMIT
- cross-user saved-card collision fixed (UNIQUE(user_id,square_card_id))
- customer provisioning (lazy, save-only); one-off/guest mint no customer
- discount preview/apply unified in discounts.go (global-milestone visible
  in preview, N+1 eliminated, redemption counter preserved on failures)
- webhook event_id dedup; refund loop dedup; stale comment fixes
- test-isolation t.Cleanup on committed sweep tests

Frontend:
- SCA tokenizeWithVerification across all charge flows (amount as
  major-units decimal), 5-min token-expiry re-tokenize, verification_token
  in request bodies
- PaymentModal synchronous double-click + zero/negative-amount guards
- till online-card UI wired to /api/admin/till/sale
- policyPopover generalised; new /privacy-policy route; consent checkbox
  copy + Square privacy link
- Square card iframe styled to app typography (Inter 14px, oklch tokens);
  mock form md:text-sm parity
- 'Secure payment powered by Square' badge on all 8 card-payment flows

Schema/docs: terminal_checkouts + square_customer_id + per-user card
constraint in init-script.sql; README migrations; P14 plan + backlog +
Technical Manual updated.

Includes 39 modified/new test files; full backend suite (25 pkgs),
-race on payments+square, and frontend build are green.

README.md

@@ -110,10 +110,40 @@ ALTER TYPE admin_notification_reason ADD VALUE IF NOT EXISTS 'refund_failed';
 
 -- Refunds may now reference non-booking payments (gift-card purchase refunds)
 ALTER TABLE refunds ALTER COLUMN booking_id DROP NOT NULL;
+
+-- Terminal checkout in-flight guard + payment_type passthrough (terminal_checkouts table)
+-- NOTE: CREATE TABLE is a fresh addition, not a column change. Apply before deploying
+-- the terminal-payment changes or CreateTerminalPayment/GetCheckoutStatus fail at runtime.
+CREATE TABLE IF NOT EXISTS terminal_checkouts (
+    checkout_id VARCHAR(64) PRIMARY KEY,
+    booking_id CHAR(12) NOT NULL REFERENCES bookings(id) ON DELETE CASCADE,
+    payment_type payment_type NOT NULL DEFAULT 'full',
+    status VARCHAR(20) NOT NULL DEFAULT 'PENDING',
+    amount NUMERIC(10,2) NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_terminal_checkouts_booking ON terminal_checkouts(booking_id, status);
+CREATE INDEX IF NOT EXISTS idx_terminal_checkouts_status ON terminal_checkouts(status);
 ```
 
 The sweep job (`internal/jobs/cleanup.go`) and `refunds.go` cast `'refund_failed'::admin_notification_reason`, so an un-migrated DB fails at runtime — apply these before deploying the payment changes.
 
+#### Saved-card per-user uniqueness + Square customer provisioning (P14)
+
+The `user_saved_cards.square_card_id` UNIQUE constraint is now scoped **per user** (`UNIQUE (user_id, square_card_id)`), so the same physical card saved by two users produces two independent rows instead of user B mutating user A's saved-card row. Existing deployments must swap the constraint (the auto-generated constraint name is `user_saved_cards_square_card_id_key`):
+
+```sql
+ALTER TABLE user_saved_cards DROP CONSTRAINT user_saved_cards_square_card_id_key;
+ALTER TABLE user_saved_cards ADD CONSTRAINT user_saved_cards_user_id_square_card_id_key UNIQUE (user_id, square_card_id);
+```
+
+`square_customer_id TEXT` (nullable) was also added to `user_saved_cards` — populated the first time a user saves a card (Square customer provisioning, P14) and reused thereafter:
+
+```sql
+ALTER TABLE user_saved_cards ADD COLUMN IF NOT EXISTS square_customer_id TEXT;
+```
+
 ## Full Documentation
 
 Detailed architecture, schema, admin workflows, user journeys, and backlog in [obsidian/Crussell/](obsidian/Crussell/).