feat: add account-based subscription model with migrations

Transition from team-based to account-based multi-tenancy (Free/Pro/Team).
Migrations 016-020 create accounts, subscriptions, plan_limits, and
account_invites tables, then migrate existing users and content FKs.

New models: Account, Subscription, PlanLimits, AccountInvite.
Updated models add account_id alongside existing team_id (coexistence
for safe two-PR deployment). Permissions and deps refactored for
account_role instead of is_team_admin.

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

backend/app/core/stripe_handlers.py

@@ -0,0 +1,37 @@
+"""Stripe webhook event handlers (stub implementations).
+
+These handlers log events but don't process them until Stripe is fully configured.
+"""
+import logging
+from sqlalchemy.ext.asyncio import AsyncSession
+
+logger = logging.getLogger(__name__)
+
+
+async def handle_checkout_completed(event: dict, db: AsyncSession) -> None:
+    logger.info("Stripe: checkout.session.completed — %s", event.get("id"))
+
+
+async def handle_invoice_paid(event: dict, db: AsyncSession) -> None:
+    logger.info("Stripe: invoice.paid — %s", event.get("id"))
+
+
+async def handle_invoice_payment_failed(event: dict, db: AsyncSession) -> None:
+    logger.warning("Stripe: invoice.payment_failed — %s", event.get("id"))
+
+
+async def handle_subscription_updated(event: dict, db: AsyncSession) -> None:
+    logger.info("Stripe: customer.subscription.updated — %s", event.get("id"))
+
+
+async def handle_subscription_deleted(event: dict, db: AsyncSession) -> None:
+    logger.info("Stripe: customer.subscription.deleted — %s", event.get("id"))
+
+
+WEBHOOK_HANDLERS = {
+    "checkout.session.completed": handle_checkout_completed,
+    "invoice.paid": handle_invoice_paid,
+    "invoice.payment_failed": handle_invoice_payment_failed,
+    "customer.subscription.updated": handle_subscription_updated,
+    "customer.subscription.deleted": handle_subscription_deleted,
+}