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/loyalty/services/loyalty_onboarding.py

@@ -0,0 +1,61 @@
+# app/modules/loyalty/services/loyalty_onboarding.py
+"""
+Onboarding provider for the loyalty module.
+
+Provides the "Create your first loyalty program" step.
+Completed when at least 1 LoyaltyProgram exists for the store's merchant.
+"""
+
+import logging
+
+from sqlalchemy.orm import Session
+
+from app.modules.contracts.onboarding import OnboardingStepDefinition
+
+logger = logging.getLogger(__name__)
+
+
+class LoyaltyOnboardingProvider:
+    """Onboarding provider for loyalty module."""
+
+    @property
+    def onboarding_category(self) -> str:
+        return "loyalty"
+
+    def get_onboarding_steps(self) -> list[OnboardingStepDefinition]:
+        return [
+            OnboardingStepDefinition(
+                key="loyalty.create_program",
+                title_key="onboarding.loyalty.create_program.title",
+                description_key="onboarding.loyalty.create_program.description",
+                icon="gift",
+                route_template="/store/{store_code}/loyalty/programs",
+                order=300,
+                category="loyalty",
+            ),
+        ]
+
+    def is_step_completed(
+        self, db: Session, store_id: int, step_key: str
+    ) -> bool:
+        if step_key != "loyalty.create_program":
+            return False
+
+        from app.modules.loyalty.models.loyalty_program import LoyaltyProgram
+        from app.modules.tenancy.models.store import Store
+
+        # Programs belong to merchant, not store — join through store
+        store = db.query(Store).filter(Store.id == store_id).first()
+        if not store:
+            return False
+
+        count = (
+            db.query(LoyaltyProgram)
+            .filter(LoyaltyProgram.merchant_id == store.merchant_id)
+            .limit(1)
+            .count()
+        )
+        return count > 0
+
+
+loyalty_onboarding_provider = LoyaltyOnboardingProvider()