Fix silent tip/gift-card money loss on pending retry; terminal checkout wire; sentinel error; docs

CRITICAL — same-amount tip retry silently never charged:
- CreateTipPayment idempotency check now only short-circuits when the
  existing record is 'completed'. A 'pending' record (previous Square call
  failed) is REUSED and the charge re-attempted with the same key (Square
  dedups safely), instead of returning the stale pending record as 200 with
  a success toast and no charge.
- Same fix in BuyGiftCard: pending records trigger a re-attempt, not a
  false-success response. Unique idempotency_key constraint means the
  pending record must be reused, not re-inserted.
- Fixes the savepoint/rollback interaction: the nested tx (savepoint) is
  now committed in the reuse path so the deferred rollback doesn't undo the
  later status UPDATE on the same connection.
- Regression test: TestTipPayment_RetryPending_ReattemptsCharge verifies a
  pending record + same-key retry re-attempts and completes, reusing the
  record (count stays 1).

MAJOR — terminal checkout wire contract:
- device_id now sent as checkout.device_options.device_id (Square's required
  shape), not a top-level field which Square rejects with 400.
- 'checkout pending' detection now uses typed sentinel ErrCheckoutPending
  with errors.Is in both handlers, matching mock and real HTTP client.

MAJOR — exp_month/exp_year omitted from card creation payload when unset
(now *int with omitempty) — Square would 400 on 0/0; expiry comes from the
tokenized source.

Docs:
- README payments/infrastructure sections corrected (Web Payments SDK claim
  replaced with accurate P11-backlog note; dev mock parity described)
- Future Work P11 updated to reflect raw-PAN rejection is now enforced in
  both mock and prod (new-card flows are a documented dead end)
- Added plans/p11-square-web-payments-sdk.md: full implementation plan +
  handoff prompt for the agent picking up P11 (Web Payments SDK nonces)

backend/internal/square/square_http_client.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -13,6 +14,12 @@ import (
 	"time"
 )
 
+// ErrCheckoutPending is returned by GetCheckout when a terminal checkout has
+// not yet completed. Handlers use errors.Is(err, ErrCheckoutPending) rather
+// than string comparison, so behaviour is identical across the mock and the
+// real HTTP client.
+var ErrCheckoutPending = errors.New("checkout pending")
+
 // ---------------------------------------------------------------------------
 // Square REST API constants.
 // ---------------------------------------------------------------------------
@@ -175,14 +182,18 @@ type sqFee struct {
 type sqTerminalCheckoutRequest struct {
 	IdempotencyKey string                    `json:"idempotency_key"`
 	Checkout       sqTerminalCheckoutPayload `json:"checkout"`
-	DeviceID       string                    `json:"device_id,omitempty"`
 }
 
 type sqTerminalCheckoutPayload struct {
-	AmountMoney sqMoney `json:"amount_money"`
-	ReferenceID string  `json:"reference_id,omitempty"`
-	Note        string  `json:"note,omitempty"`
-	CustomerID  string  `json:"customer_id,omitempty"`
+	AmountMoney  sqMoney                 `json:"amount_money"`
+	ReferenceID  string                  `json:"reference_id,omitempty"`
+	Note         string                  `json:"note,omitempty"`
+	CustomerID   string                  `json:"customer_id,omitempty"`
+	DeviceOptions *sqDeviceOptions       `json:"device_options,omitempty"`
+}
+
+type sqDeviceOptions struct {
+	DeviceID string `json:"device_id"`
 }
 
 type sqTerminalCheckoutResponse struct {
@@ -238,8 +249,8 @@ type sqCreateCardRequest struct {
 }
 
 type sqCardPayload struct {
-	ExpMonth       int    `json:"exp_month"`
-	ExpYear        int    `json:"exp_year"`
+	ExpMonth       *int   `json:"exp_month,omitempty"`
+	ExpYear        *int   `json:"exp_year,omitempty"`
 	CardholderName string `json:"cardholder_name,omitempty"`
 	CustomerID     string `json:"customer_id,omitempty"`
 }
@@ -295,7 +306,9 @@ func createCheckoutHTTP(ctx context.Context, req CreateCheckoutReq) (*CheckoutRe
 			Note:        req.Note,
 			CustomerID:  req.CustomerID,
 		},
-		DeviceID: req.DeviceID,
+	}
+	if req.DeviceID != "" {
+		body.Checkout.DeviceOptions = &sqDeviceOptions{DeviceID: req.DeviceID}
 	}
 	var resp sqTerminalCheckoutResponse
 	if err := hc.doJSON(ctx, http.MethodPost, "/v2/terminals/checkouts", body, &resp); err != nil {
@@ -312,6 +325,9 @@ func getCheckoutHTTP(ctx context.Context, checkoutID string) (*PaymentResult, er
 	}
 	tc := tcResp.Checkout
 	if tc.Status != "COMPLETED" {
+		if tc.Status == "PENDING" || tc.Status == "IN_PROGRESS" {
+			return nil, ErrCheckoutPending
+		}
 		return nil, fmt.Errorf("square: checkout %s is %s (not COMPLETED)", checkoutID, tc.Status)
 	}
 	if len(tc.PaymentIDs) == 0 {
@@ -351,8 +367,6 @@ func createCardOnFileHTTP(ctx context.Context, userID, cardToken string) (*CardO
 		SourceID:       cardToken,
 		Card: sqCardPayload{
 			CustomerID: userID,
-			ExpMonth:   0,
-			ExpYear:    0,
 		},
 	}
 	var resp sqCreateCardResponse