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/contracts/__init__.py

@@ -63,6 +63,11 @@ from app.modules.contracts.metrics import (
     MetricsProviderProtocol,
     MetricValue,
 )
+from app.modules.contracts.onboarding import (
+    OnboardingProviderProtocol,
+    OnboardingStepDefinition,
+    OnboardingStepStatus,
+)
 from app.modules.contracts.widgets import (
     BreakdownWidget,
     DashboardWidget,
@@ -92,6 +97,10 @@ __all__ = [
     "MetricValue",
     "MetricsContext",
     "MetricsProviderProtocol",
+    # Onboarding protocols
+    "OnboardingStepDefinition",
+    "OnboardingStepStatus",
+    "OnboardingProviderProtocol",
     # Widget protocols
     "WidgetContext",
     "WidgetListItem",