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/modules/billing/routes/pages/platform.py

@@ -8,7 +8,7 @@ Platform (unauthenticated) pages for pricing and signup:
 - Signup success
 """
 
-from fastapi import APIRouter, Depends, Request
+from fastapi import APIRouter, Depends, HTTPException, Request
 from fastapi.responses import HTMLResponse
 from sqlalchemy.orm import Session
 
@@ -16,22 +16,30 @@ from app.core.database import get_db
 from app.modules.core.utils.page_context import get_platform_context
 from app.templates_config import templates
 
+
+def _require_platform(request: Request):
+    """Get the current platform or raise 404. Platform must always be known."""
+    platform = getattr(request.state, "platform", None)
+    if not platform:
+        raise HTTPException(
+            status_code=404,
+            detail="Platform not detected. Pricing and signup require a known platform.",
+        )
+    return platform
+
 router = APIRouter()
 
 
-def _get_tiers_data(db: Session) -> list[dict]:
+def _get_tiers_data(db: Session, platform_id: int) -> list[dict]:
     """Build tier data for display in templates from database."""
     from app.modules.billing.models import SubscriptionTier, TierCode
 
-    tiers_db = (
-        db.query(SubscriptionTier)
-        .filter(
-            SubscriptionTier.is_active == True,
-            SubscriptionTier.is_public == True,
-        )
-        .order_by(SubscriptionTier.display_order)
-        .all()
+    query = db.query(SubscriptionTier).filter(
+        SubscriptionTier.is_active == True,
+        SubscriptionTier.is_public == True,
+        SubscriptionTier.platform_id == platform_id,
     )
+    tiers_db = query.order_by(SubscriptionTier.display_order).all()
 
     tiers = []
     for tier in tiers_db:
@@ -63,9 +71,12 @@ async def pricing_page(
 ):
     """
     Standalone pricing page with detailed tier comparison.
+
+    Tiers are filtered by the current platform (detected from domain/path).
     """
+    platform = _require_platform(request)
     context = get_platform_context(request, db)
-    context["tiers"] = _get_tiers_data(db)
+    context["tiers"] = _get_tiers_data(db, platform_id=platform.id)
     context["page_title"] = "Pricing"
 
     return templates.TemplateResponse(
@@ -89,18 +100,28 @@ async def signup_page(
     """
     Multi-step signup wizard.
 
+    Routes to platform-specific signup templates. Each platform defines
+    its own signup flow (different steps, different UI).
+
     Query params:
     - tier: Pre-selected tier code
     - annual: Pre-select annual billing
     """
+    platform = _require_platform(request)
     context = get_platform_context(request, db)
     context["page_title"] = "Start Your Free Trial"
     context["selected_tier"] = tier
     context["is_annual"] = annual
-    context["tiers"] = _get_tiers_data(db)
+    context["tiers"] = _get_tiers_data(db, platform_id=platform.id)
+
+    # Route to platform-specific signup template
+    if platform.code == "loyalty":
+        template_name = "billing/platform/signup-loyalty.html"
+    else:
+        template_name = "billing/platform/signup.html"
 
     return templates.TemplateResponse(
-        "billing/platform/signup.html",
+        template_name,
         context,
     )