feat: module-driven onboarding system + simplified 3-step signup

Add OnboardingProviderProtocol so modules declare their own post-signup
onboarding steps. The core OnboardingAggregator discovers enabled
providers and exposes a dashboard API (GET /dashboard/onboarding).
A session-scoped banner on the store dashboard shows a checklist that
guides merchants through setup without blocking signup.

Signup is simplified from 4 steps to 3 (Plan → Account → Payment):
store creation is merged into account creation, store language is
captured from the user's browsing language, and platform-specific
template branching is removed.

Includes 47 unit and integration tests covering all new providers,
the aggregator, the API endpoint, and the signup service changes.

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

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

@@ -114,14 +114,8 @@ async def signup_page(
     context["is_annual"] = annual
     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(
-        template_name,
+        "billing/platform/signup.html",
         context,
     )