feat(billing): add BillingService.start_trial; wire into /auth/register

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/endpoints/auth.py

@@ -195,26 +195,30 @@ async def register(
         # Now set account owner and create subscription
         new_account.owner_id = new_user.id
 
-        # Apply plan/trial from invite code if present
-        sub_plan = "free"
-        sub_status = "active"
-        period_start = None
-        period_end = None
         if invite_code_record and invite_code_record.assigned_plan:
+            # Plan/trial driven by platform invite code (existing pilot flow)
             sub_plan = invite_code_record.assigned_plan
+            sub_status = "active"
+            period_start = None
+            period_end = None
             if invite_code_record.trial_duration_days:
                 sub_status = "trialing"
                 period_start = datetime.now(timezone.utc)
                 period_end = period_start + timedelta(days=invite_code_record.trial_duration_days)
-
-        new_subscription = Subscription(
-            account_id=new_account.id,
-            plan=sub_plan,
-            status=sub_status,
-            current_period_start=period_start,
-            current_period_end=period_end,
-        )
-        db.add(new_subscription)
+            db.add(Subscription(
+                account_id=new_account.id,
+                plan=sub_plan,
+                status=sub_status,
+                current_period_start=period_start,
+                current_period_end=period_end,
+            ))
+        else:
+            # New self-serve shop — start the standard Pro trial.
+            # start_trial commits internally; flush our pending User/Account changes
+            # first so the FK is satisfied.
+            await db.flush()
+            from app.services.billing import BillingService
+            await BillingService.start_trial(db, new_account.id)
 
     # Mark platform invite code as used
     if invite_code_record:

backend/app/services/billing.py

@@ -0,0 +1,36 @@
+"""Single billing service module. Stripe is the only impl — no provider
+abstraction. Account row is canonical local state; Stripe is canonical
+remote state; the webhook handler bridges the two."""
+from datetime import datetime, timezone, timedelta
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.subscription import Subscription
+
+
+TRIAL_DAYS = 14
+
+
+class BillingService:
+    @staticmethod
+    async def start_trial(db: AsyncSession, account_id) -> Subscription:
+        """Idempotent. Creates a trialing Subscription on Pro for the account if
+        one doesn't exist; otherwise returns the existing row."""
+        result = await db.execute(
+            select(Subscription).where(Subscription.account_id == account_id)
+        )
+        existing = result.scalar_one_or_none()
+        if existing is not None:
+            return existing
+
+        sub = Subscription(
+            account_id=account_id,
+            plan="pro",
+            status="trialing",
+            current_period_start=datetime.now(timezone.utc),
+            current_period_end=datetime.now(timezone.utc) + timedelta(days=TRIAL_DAYS),
+        )
+        db.add(sub)
+        await db.commit()
+        await db.refresh(sub)
+        return sub