feat(billing): end-to-end Stripe subscription signup with platform enforcement

Move core signup service from marketplace to billing module, add
automatic Stripe product/price sync for tiers, create loyalty-specific
signup wizard, and enforce that platform is always explicitly known
(no silent defaulting to primary/hardcoded ID).

Key changes:
- New billing SignupService with separated account/store creation steps
- Stripe auto-sync on tier create/update (new prices, archive old)
- Loyalty signup template (Plan → Account → Store → Payment)
- platform_code is now required throughout the signup flow
- Pricing/signup pages return 404 if platform not detected
- OMS-specific logic (Letzshop claiming) stays in marketplace module
- Bootstrap script: scripts/seed/sync_stripe_products.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/v1/platform/signup.py

@@ -3,11 +3,14 @@
 Platform signup API endpoints.
 
 Handles the multi-step signup flow:
-1. Start signup (select tier)
-2. Claim Letzshop store (optional)
-3. Create account
+1. Start signup (select tier + platform)
+2. Create account (user + merchant)
+3. Create store
 4. Setup payment (collect card via SetupIntent)
-5. Complete signup (create subscription with trial)
+5. Complete signup (create Stripe subscription with trial)
+
+Platform-specific steps (e.g., OMS Letzshop claiming) are handled
+by their respective modules and call into this core flow.
 
 All endpoints are public (no authentication required).
 """
@@ -20,9 +23,7 @@ from sqlalchemy.orm import Session
 
 from app.core.database import get_db
 from app.core.environment import should_use_secure_cookies
-from app.modules.marketplace.services.platform_signup_service import (
-    platform_signup_service,
-)
+from app.modules.billing.services.signup_service import signup_service
 
 router = APIRouter()
 logger = logging.getLogger(__name__)
@@ -34,10 +35,11 @@ logger = logging.getLogger(__name__)
 
 
 class SignupStartRequest(BaseModel):
-    """Start signup - select tier."""
+    """Start signup - select tier and platform."""
 
     tier_code: str
     is_annual: bool = False
+    platform_code: str
 
 
 class SignupStartResponse(BaseModel):
@@ -46,26 +48,11 @@ class SignupStartResponse(BaseModel):
     session_id: str
     tier_code: str
     is_annual: bool
-
-
-class ClaimStoreRequest(BaseModel):
-    """Claim Letzshop store."""
-
-    session_id: str
-    letzshop_slug: str
-    letzshop_store_id: str | None = None
-
-
-class ClaimStoreResponse(BaseModel):
-    """Response from store claim."""
-
-    session_id: str
-    letzshop_slug: str
-    store_name: str | None
+    platform_code: str
 
 
 class CreateAccountRequest(BaseModel):
-    """Create account."""
+    """Create account (user + merchant)."""
 
     session_id: str
     email: EmailStr
@@ -81,10 +68,26 @@ class CreateAccountResponse(BaseModel):
 
     session_id: str
     user_id: int
-    store_id: int
+    merchant_id: int
     stripe_customer_id: str
 
 
+class CreateStoreRequest(BaseModel):
+    """Create store for the merchant."""
+
+    session_id: str
+    store_name: str | None = None
+    language: str | None = None
+
+
+class CreateStoreResponse(BaseModel):
+    """Response from store creation."""
+
+    session_id: str
+    store_id: int
+    store_code: str
+
+
 class SetupPaymentRequest(BaseModel):
     """Request payment setup."""
 
@@ -127,43 +130,20 @@ async def start_signup(request: SignupStartRequest) -> SignupStartResponse:
     """
     Start the signup process.
 
-    Step 1: User selects a tier and billing period.
+    Step 1: User selects a tier, billing period, and platform.
     Creates a signup session to track the flow.
     """
-    session_id = platform_signup_service.create_session(
+    session_id = signup_service.create_session(
         tier_code=request.tier_code,
         is_annual=request.is_annual,
+        platform_code=request.platform_code,
     )
 
     return SignupStartResponse(
         session_id=session_id,
         tier_code=request.tier_code,
         is_annual=request.is_annual,
-    )
-
-
-@router.post("/signup/claim-store", response_model=ClaimStoreResponse)  # public
-async def claim_letzshop_store(
-    request: ClaimStoreRequest,
-    db: Session = Depends(get_db),
-) -> ClaimStoreResponse:
-    """
-    Claim a Letzshop store.
-
-    Step 2 (optional): User claims their Letzshop shop.
-    This pre-fills store info during account creation.
-    """
-    store_name = platform_signup_service.claim_store(
-        db=db,
-        session_id=request.session_id,
-        letzshop_slug=request.letzshop_slug,
-        letzshop_store_id=request.letzshop_store_id,
-    )
-
-    return ClaimStoreResponse(
-        session_id=request.session_id,
-        letzshop_slug=request.letzshop_slug,
-        store_name=store_name,
+        platform_code=request.platform_code,
     )
 
 
@@ -173,12 +153,13 @@ async def create_account(
     db: Session = Depends(get_db),
 ) -> CreateAccountResponse:
     """
-    Create user and store accounts.
+    Create user and merchant accounts.
 
-    Step 3: User provides account details.
-    Creates User, Merchant, Store, and Stripe Customer.
+    Step 2: User provides account details.
+    Creates User, Merchant, and Stripe Customer.
+    Store creation is a separate step.
     """
-    result = platform_signup_service.create_account(
+    result = signup_service.create_account(
         db=db,
         session_id=request.session_id,
         email=request.email,
@@ -192,11 +173,36 @@ async def create_account(
     return CreateAccountResponse(
         session_id=request.session_id,
         user_id=result.user_id,
-        store_id=result.store_id,
+        merchant_id=result.merchant_id,
         stripe_customer_id=result.stripe_customer_id,
     )
 
 
+@router.post("/signup/create-store", response_model=CreateStoreResponse)  # public
+async def create_store(
+    request: CreateStoreRequest,
+    db: Session = Depends(get_db),
+) -> CreateStoreResponse:
+    """
+    Create the first store for the merchant.
+
+    Step 3: User names their store (defaults to merchant name).
+    Creates Store, StorePlatform, and MerchantSubscription.
+    """
+    result = signup_service.create_store(
+        db=db,
+        session_id=request.session_id,
+        store_name=request.store_name,
+        language=request.language,
+    )
+
+    return CreateStoreResponse(
+        session_id=request.session_id,
+        store_id=result.store_id,
+        store_code=result.store_code,
+    )
+
+
 @router.post("/signup/setup-payment", response_model=SetupPaymentResponse)  # public
 async def setup_payment(request: SetupPaymentRequest) -> SetupPaymentResponse:
     """
@@ -205,7 +211,7 @@ async def setup_payment(request: SetupPaymentRequest) -> SetupPaymentResponse:
     Step 4: Collect card details without charging.
     The card will be charged after the trial period ends.
     """
-    client_secret, stripe_customer_id = platform_signup_service.setup_payment(
+    client_secret, stripe_customer_id = signup_service.setup_payment(
         session_id=request.session_id,
     )
 
@@ -228,7 +234,7 @@ async def complete_signup(
     Step 5: Verify SetupIntent, attach payment method, create subscription.
     Also sets HTTP-only cookie for page navigation and returns token for localStorage.
     """
-    result = platform_signup_service.complete_signup(
+    result = signup_service.complete_signup(
         db=db,
         session_id=request.session_id,
         setup_intent_id=request.setup_intent_id,
@@ -265,7 +271,7 @@ async def get_signup_session(session_id: str) -> dict:
 
     Useful for resuming an incomplete signup.
     """
-    session = platform_signup_service.get_session_or_raise(session_id)
+    session = signup_service.get_session_or_raise(session_id)
 
     # Return safe subset of session data
     return {