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/api/v1/platform/signup.py

@@ -40,6 +40,7 @@ class SignupStartRequest(BaseModel):
     tier_code: str
     is_annual: bool = False
     platform_code: str
+    language: str = "fr"
 
 
 class SignupStartResponse(BaseModel):
@@ -64,12 +65,14 @@ class CreateAccountRequest(BaseModel):
 
 
 class CreateAccountResponse(BaseModel):
-    """Response from account creation."""
+    """Response from account creation (includes auto-created store)."""
 
     session_id: str
     user_id: int
     merchant_id: int
     stripe_customer_id: str
+    store_id: int
+    store_code: str
 
 
 class CreateStoreRequest(BaseModel):
@@ -137,6 +140,7 @@ async def start_signup(request: SignupStartRequest) -> SignupStartResponse:
         tier_code=request.tier_code,
         is_annual=request.is_annual,
         platform_code=request.platform_code,
+        language=request.language,
     )
 
     return SignupStartResponse(
@@ -175,6 +179,8 @@ async def create_account(
         user_id=result.user_id,
         merchant_id=result.merchant_id,
         stripe_customer_id=result.stripe_customer_id,
+        store_id=result.store_id,
+        store_code=result.store_code,
     )