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/base.py

@@ -49,6 +49,7 @@ if TYPE_CHECKING:
     from app.modules.contracts.cms import MediaUsageProviderProtocol
     from app.modules.contracts.features import FeatureProviderProtocol
     from app.modules.contracts.metrics import MetricsProviderProtocol
+    from app.modules.contracts.onboarding import OnboardingProviderProtocol
     from app.modules.contracts.widgets import DashboardWidgetProviderProtocol
 
 from app.modules.enums import FrontendType
@@ -486,6 +487,29 @@ class ModuleDefinition:
     # to report where media is being used.
     media_usage_provider: "Callable[[], MediaUsageProviderProtocol] | None" = None
 
+    # =========================================================================
+    # Onboarding Provider (Module-Driven Post-Signup Onboarding)
+    # =========================================================================
+    # Callable that returns an OnboardingProviderProtocol implementation.
+    # Modules declare onboarding steps (what needs to be configured after signup)
+    # and provide completion checks. The core module's OnboardingAggregator
+    # discovers and aggregates all providers into a dashboard checklist banner.
+    #
+    # Example:
+    #     def _get_onboarding_provider():
+    #         from app.modules.marketplace.services.marketplace_onboarding import (
+    #             marketplace_onboarding_provider,
+    #         )
+    #         return marketplace_onboarding_provider
+    #
+    #     marketplace_module = ModuleDefinition(
+    #         code="marketplace",
+    #         onboarding_provider=_get_onboarding_provider,
+    #     )
+    #
+    # The provider will be discovered by core's OnboardingAggregator service.
+    onboarding_provider: "Callable[[], OnboardingProviderProtocol] | None" = None
+
     # =========================================================================
     # Menu Item Methods (Legacy - uses menu_items dict of IDs)
     # =========================================================================
@@ -955,6 +979,24 @@ class ModuleDefinition:
             return None
         return self.media_usage_provider()
 
+    # =========================================================================
+    # Onboarding Provider Methods
+    # =========================================================================
+
+    def has_onboarding_provider(self) -> bool:
+        """Check if this module has an onboarding provider."""
+        return self.onboarding_provider is not None
+
+    def get_onboarding_provider_instance(self) -> "OnboardingProviderProtocol | None":
+        """Get the onboarding provider instance for this module.
+
+        Returns:
+            OnboardingProviderProtocol instance, or None
+        """
+        if self.onboarding_provider is None:
+            return None
+        return self.onboarding_provider()
+
     # =========================================================================
     # Magic Methods
     # =========================================================================