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/tenancy/definition.py

@@ -36,6 +36,15 @@ def _get_feature_provider():
     return tenancy_feature_provider
 
 
+def _get_onboarding_provider():
+    """Lazy import of onboarding provider to avoid circular imports."""
+    from app.modules.tenancy.services.tenancy_onboarding import (
+        tenancy_onboarding_provider,
+    )
+
+    return tenancy_onboarding_provider
+
+
 tenancy_module = ModuleDefinition(
     code="tenancy",
     name="Tenancy Management",
@@ -233,6 +242,7 @@ tenancy_module = ModuleDefinition(
     # Widget provider for dashboard widgets
     widget_provider=_get_widget_provider,
     feature_provider=_get_feature_provider,
+    onboarding_provider=_get_onboarding_provider,
 )
 
 __all__ = ["tenancy_module"]