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/tests/unit/test_marketplace_onboarding.py

@@ -0,0 +1,140 @@
+# app/modules/marketplace/tests/unit/test_marketplace_onboarding.py
+"""Unit tests for MarketplaceOnboardingProvider."""
+
+import uuid
+
+import pytest
+
+from app.modules.marketplace.services.marketplace_onboarding import (
+    MarketplaceOnboardingProvider,
+)
+from app.modules.tenancy.models import Merchant, Store, User
+
+
+@pytest.fixture
+def mp_owner(db):
+    """Create a store owner for marketplace onboarding tests."""
+    from middleware.auth import AuthManager
+
+    auth = AuthManager()
+    user = User(
+        email=f"mpowner_{uuid.uuid4().hex[:8]}@test.com",
+        username=f"mpowner_{uuid.uuid4().hex[:8]}",
+        hashed_password=auth.hash_password("pass123"),
+        role="merchant_owner",
+        is_active=True,
+    )
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+    return user
+
+
+@pytest.fixture
+def mp_merchant(db, mp_owner):
+    """Create a merchant for marketplace onboarding tests."""
+    merchant = Merchant(
+        name="MP Onboarding Merchant",
+        owner_user_id=mp_owner.id,
+        contact_email=mp_owner.email,
+        is_active=True,
+        is_verified=True,
+    )
+    db.add(merchant)
+    db.commit()
+    db.refresh(merchant)
+    return merchant
+
+
+@pytest.fixture
+def mp_store(db, mp_merchant):
+    """Create a store for marketplace onboarding tests."""
+    uid = uuid.uuid4().hex[:8]
+    store = Store(
+        merchant_id=mp_merchant.id,
+        store_code=f"MPSTORE_{uid.upper()}",
+        subdomain=f"mpstore{uid.lower()}",
+        name="MP Test Store",
+        is_active=True,
+        is_verified=True,
+    )
+    db.add(store)
+    db.commit()
+    db.refresh(store)
+    return store
+
+
+@pytest.mark.unit
+@pytest.mark.marketplace
+class TestMarketplaceOnboardingProvider:
+    """Tests for MarketplaceOnboardingProvider."""
+
+    def setup_method(self):
+        self.provider = MarketplaceOnboardingProvider()
+
+    def test_category(self):
+        """Returns 'marketplace' as the onboarding category."""
+        assert self.provider.onboarding_category == "marketplace"
+
+    def test_get_onboarding_steps_returns_two_steps(self):
+        """Returns two steps: connect_api and import_products."""
+        steps = self.provider.get_onboarding_steps()
+        assert len(steps) == 2
+        assert steps[0].key == "marketplace.connect_api"
+        assert steps[0].order == 200
+        assert steps[1].key == "marketplace.import_products"
+        assert steps[1].order == 210
+
+    def test_connect_api_incomplete_without_credentials(self, db, mp_store):
+        """connect_api step is incomplete when no credentials exist."""
+        result = self.provider.is_step_completed(
+            db, mp_store.id, "marketplace.connect_api"
+        )
+        assert result is False
+
+    def test_connect_api_complete_with_credentials(self, db, mp_store):
+        """connect_api step is complete when StoreLetzshopCredentials has api_key."""
+        from app.modules.marketplace.models.letzshop import StoreLetzshopCredentials
+
+        creds = StoreLetzshopCredentials(
+            store_id=mp_store.id,
+            api_key_encrypted="encrypted_key_here",
+        )
+        db.add(creds)
+        db.commit()
+
+        result = self.provider.is_step_completed(
+            db, mp_store.id, "marketplace.connect_api"
+        )
+        assert result is True
+
+    def test_import_products_incomplete_without_products(self, db, mp_store):
+        """import_products step is incomplete when store has no products."""
+        result = self.provider.is_step_completed(
+            db, mp_store.id, "marketplace.import_products"
+        )
+        assert result is False
+
+    def test_import_products_complete_with_products(self, db, mp_store):
+        """import_products step is complete when store has at least one product."""
+        from app.modules.catalog.models.product import Product
+
+        product = Product(
+            store_id=mp_store.id,
+            store_sku=f"SKU-{uuid.uuid4().hex[:8]}",
+            is_active=True,
+        )
+        db.add(product)
+        db.commit()
+
+        result = self.provider.is_step_completed(
+            db, mp_store.id, "marketplace.import_products"
+        )
+        assert result is True
+
+    def test_unknown_step_key_returns_false(self, db, mp_store):
+        """Returns False for unrecognized step key."""
+        result = self.provider.is_step_completed(
+            db, mp_store.id, "marketplace.unknown"
+        )
+        assert result is False