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/services/marketplace_onboarding.py

@@ -0,0 +1,79 @@
+# app/modules/marketplace/services/marketplace_onboarding.py
+"""
+Onboarding provider for the marketplace module.
+
+Provides two onboarding steps:
+1. Connect Letzshop API - completed when StoreLetzshopCredentials exists with api_key
+2. Import products - completed when at least 1 product exists in catalog
+"""
+
+import logging
+
+from sqlalchemy.orm import Session
+
+from app.modules.contracts.onboarding import OnboardingStepDefinition
+
+logger = logging.getLogger(__name__)
+
+
+class MarketplaceOnboardingProvider:
+    """Onboarding provider for marketplace module."""
+
+    @property
+    def onboarding_category(self) -> str:
+        return "marketplace"
+
+    def get_onboarding_steps(self) -> list[OnboardingStepDefinition]:
+        return [
+            OnboardingStepDefinition(
+                key="marketplace.connect_api",
+                title_key="onboarding.marketplace.connect_api.title",
+                description_key="onboarding.marketplace.connect_api.description",
+                icon="plug",
+                route_template="/store/{store_code}/letzshop",
+                order=200,
+                category="marketplace",
+            ),
+            OnboardingStepDefinition(
+                key="marketplace.import_products",
+                title_key="onboarding.marketplace.import_products.title",
+                description_key="onboarding.marketplace.import_products.description",
+                icon="package",
+                route_template="/store/{store_code}/marketplace",
+                order=210,
+                category="marketplace",
+            ),
+        ]
+
+    def is_step_completed(
+        self, db: Session, store_id: int, step_key: str
+    ) -> bool:
+        if step_key == "marketplace.connect_api":
+            return self._has_letzshop_credentials(db, store_id)
+        if step_key == "marketplace.import_products":
+            return self._has_products(db, store_id)
+        return False
+
+    def _has_letzshop_credentials(self, db: Session, store_id: int) -> bool:
+        from app.modules.marketplace.models.letzshop import StoreLetzshopCredentials
+
+        creds = (
+            db.query(StoreLetzshopCredentials)
+            .filter(StoreLetzshopCredentials.store_id == store_id)
+            .first()
+        )
+        return bool(creds and creds.api_key_encrypted)
+
+    def _has_products(self, db: Session, store_id: int) -> bool:
+        from app.modules.catalog.models.product import Product
+
+        count = (
+            db.query(Product)
+            .filter(Product.store_id == store_id)
+            .limit(1)
+            .count()
+        )
+        return count > 0
+
+
+marketplace_onboarding_provider = MarketplaceOnboardingProvider()