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/marketplace/routes/pages/store.py

@@ -95,15 +95,7 @@ async def store_marketplace_page(
     """
     Render marketplace import page.
     JavaScript loads import jobs and products via API.
-    Redirects to onboarding if not completed.
     """
-    onboarding_service = OnboardingService(db)
-    if not onboarding_service.is_completed(current_user.token_store_id):
-        return RedirectResponse(
-            url=f"/store/{store_code}/onboarding",
-            status_code=302,
-        )
-
     return templates.TemplateResponse(
         "marketplace/store/marketplace.html",
         get_store_context(request, db, current_user, store_code),
@@ -127,15 +119,7 @@ async def store_letzshop_page(
     """
     Render Letzshop integration page.
     JavaScript loads orders, credentials status, and handles fulfillment operations.
-    Redirects to onboarding if not completed.
     """
-    onboarding_service = OnboardingService(db)
-    if not onboarding_service.is_completed(current_user.token_store_id):
-        return RedirectResponse(
-            url=f"/store/{store_code}/onboarding",
-            status_code=302,
-        )
-
     return templates.TemplateResponse(
         "marketplace/store/letzshop.html",
         get_store_context(request, db, current_user, store_code),