feat: update all endpoints and schemas for account-based model

Replace team_id with account_id across all API endpoints (trees,
categories, tags, steps, step_categories, admin, auth). Add new
accounts and webhooks endpoints. Registration now atomically creates
Account + Subscription, with account_invite_code bypassing the
platform invite gate.

Schemas updated for account_id/account_role. 82 tests passing
including 18 new tests for accounts, subscriptions, and permissions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/webhooks.py

@@ -0,0 +1,62 @@
+import logging
+from fastapi import APIRouter, Request, HTTPException, status, Depends
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.database import get_db
+from app.core.config import settings
+from app.core.stripe_handlers import WEBHOOK_HANDLERS
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/webhooks", tags=["webhooks"])
+
+
+@router.post("/stripe")
+async def stripe_webhook(
+    request: Request,
+    db: AsyncSession = Depends(get_db),
+):
+    """Handle Stripe webhook events.
+
+    Returns 200 for all events to prevent Stripe retries.
+    Actual processing happens only when Stripe is configured.
+    """
+    if not settings.stripe_enabled:
+        return {"status": "ok", "message": "Stripe not configured, event ignored"}
+
+    payload = await request.body()
+    sig_header = request.headers.get("stripe-signature")
+
+    if not sig_header:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Missing stripe-signature header"
+        )
+
+    # Verify webhook signature
+    try:
+        import stripe
+        stripe.api_key = settings.STRIPE_SECRET_KEY
+        event = stripe.Webhook.construct_event(
+            payload, sig_header, settings.STRIPE_WEBHOOK_SECRET
+        )
+    except ImportError:
+        logger.warning("stripe package not installed, cannot verify webhook")
+        return {"status": "ok", "message": "stripe package not installed"}
+    except Exception as e:
+        logger.error("Stripe webhook signature verification failed: %s", e)
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Invalid signature"
+        )
+
+    event_type = event.get("type", "")
+    handler = WEBHOOK_HANDLERS.get(event_type)
+
+    if handler:
+        try:
+            await handler(event, db)
+        except Exception:
+            logger.exception("Error handling Stripe event %s", event_type)
+
+    return {"status": "ok"}