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/core/routes/api/store_dashboard.py

@@ -24,6 +24,7 @@ from app.modules.core.schemas.dashboard import (
     StoreProductStats,
     StoreRevenueStats,
 )
+from app.modules.core.services.onboarding_aggregator import onboarding_aggregator
 from app.modules.core.services.stats_aggregator import stats_aggregator
 from app.modules.tenancy.exceptions import StoreNotActiveException
 from app.modules.tenancy.schemas.auth import UserContext
@@ -124,3 +125,27 @@ def get_store_dashboard_stats(
             this_month=float(revenue_this_month),
         ),
     )
+
+
+@store_dashboard_router.get("/onboarding")
+def get_onboarding_status(
+    request: Request,
+    current_user: UserContext = Depends(get_current_store_api),
+    platform=Depends(require_platform),
+    db: Session = Depends(get_db),
+):
+    """
+    Get onboarding checklist status for the current store.
+
+    Returns steps from all enabled modules with completion status,
+    progress percentage, and whether all steps are completed.
+    """
+    store_id = current_user.token_store_id
+    store = store_service.get_store_by_id(db, store_id)
+
+    return onboarding_aggregator.get_onboarding_summary(
+        db=db,
+        store_id=store_id,
+        platform_id=platform.id,
+        store_code=store.store_code,
+    )