fix: review-loop B — adversarial findings (sweep auto-refund, admin clamp, 2FA real challenge, opaque refresh tokens, gated client IP, GBP pence)

Loop B aggressive adversarial round (3 attack agents) + fix + secondary + verification:
- CRITICAL: sweep replay auto-refunds provably-created-later duplicate charges (gated on parseable CreatedAt); 22h legitimate-retry window == 22h sweep cutoff (no dead zone)
- HIGH: admin Take Payment clamps to remaining obligation (cash/giftcard/saved-card/terminal); no unintended tip from overflow; campaign credit against remaining
- HIGH: /api/services/eligible-for/{id} requires auth + owner-or-admin (DOB/age + patch-test health-data leak closed)
- HIGH: opaque refresh-token rotation (login/refresh return {token, jti, refreshToken}; refresh REQUIRES opaque token; single-use rotation; logout revokes; access token rejected at refresh)
- HIGH: saved-card charges require a REAL 2FA verification code (B6/B10) — backend gate on all 8 charge paths + shared TwoFactorCodeInput frontend component on all 7 surfaces; 2FA gate is no longer setup-flag-only
- MEDIUM: ungated CF-Connecting-IP in reserve/admin_reserve gated via exported mw.ClientIP; 2FA limiter keyed on userID alone (no header-rotation bypass); ChangePassword actually revokes JTI + refresh tokens; 2FA setup mint cooldown + persistent failed-attempt counter; campaign redemption race surfaces campaign_fully_redeemed
- Terminal saved-card VAT applied (was under-collected); age-guard reconcile failures notify; isWeakJWTSecret entropy gate; gift-card redeem per-card counter + per-user limiter; webhook signature key startup validation
- NEW internal/twofa package (single source of truth breaking the payments<->user import cycle); consolidation of duplicate 2FA hash/verify
- Frontend: refresh-token storage + rotation, TwoFactorCodeInput component, amountPaidPence in admin modal, B5/B6/B10 contract wiring; 70 frontend tests
- Tests: loop_b_fixes_test.go, internal/twofa tests, updated auth/services/profile/twofa/mw tests

All 26 backend packages pass (incl. internal/twofa); frontend 70/70 + build clean; env-docs 41/41.

backend/auth/jwt.go

@@ -19,11 +19,16 @@ import (
 
 var TokenAuth *jwtauth.JWTAuth
 
-// AuthResponse is the response structure for login/refresh endpoints
+// AuthResponse is the response structure for login/refresh endpoints.
+// RefreshToken is a 90-day opaque, DB-hashed, single-use credential; the
+// client stores it and presents it (Bearer) to POST /api/refresh-token in
+// exchange for a fresh access token + a rotated refresh token. It is returned
+// in JSON so the SPA can persist it — omitting it would make the refresh flow
+// unusable — but it is never logged and never returned by any other endpoint.
 type AuthResponse struct {
 	Token        string `json:"token"`
 	JTI          string `json:"jti"`
-	RefreshToken string `json:"-"`
+	RefreshToken string `json:"refreshToken,omitempty"`
 }
 
 // generateJTI generates a UUID v4 string using crypto/rand