fix: restart-loop-A findings — pending sweep refunds, tip carve on discounts, TOCTOU redemption, single-use 2FA code + mint endpoint, refresh-token family revocation, admin 2FA code UX

Restart of Loop A (fresh review -> fix -> verify) findings from commit 5e967fa:
- B1: sweep auto-refund treats Square PENDING refunds as NON-terminal (row stays pending, no gift-card clawback, refunds row inserted for payments AND till_sales, re-polls the deterministic sweepdup- key); Square-less pre-pass exempts square_refund_id IS NOT NULL rows
- M4: terminal tip carve accounts for pending campaign discounts (headroom = total - pending - paid) so explicit tips aren't absorbed as service revenue; no-tip case stays a single record
- max_redemptions TOCTOU closed with atomic conditional UPDATE ... RETURNING; exhausted-at-apply surfaces campaign_fully_redeemed
- 2FA: verification code is single-use on the saved-card gate (VerifyForUser consume=true, interactive flows unaffected); new POST /api/user/2fa/code mints a fresh code for enabled users (RequireAuth + RequireNonGuest + mint cooldown + per-user limiter)
- Refresh tokens: family_id + used_at columns; reuse of an already-rotated token revokes the ENTIRE family and inserts a refresh_token_reuse admin alert; rotation mints descendants in the same family
- Frontend: 2FA code input + Request-a-new-code on all saved-card surfaces; admin modal keys code input to customer 2FA + 403 self-heal; tip-display note for pending discounts; 76 frontend tests
- Verified: all 26 backend packages pass, frontend build+tests green, env-docs 41/41

backend/handlers/user/profile_test.go

@@ -213,7 +213,7 @@ func TestPasswordChange_RevokesTokens(t *testing.T) {
 	}
 
 	// The consumed refresh token no longer verifies.
-	if _, _, err := auth.VerifyRefreshToken(ctx, refreshToken); err == nil {
+	if _, _, _, err := auth.VerifyRefreshToken(ctx, refreshToken); err == nil {
 		t.Error("refresh token must be invalid after a password change (B9)")
 	}
 }