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/models/__init__.py

@@ -1,5 +1,9 @@
 from .user import User
 from .team import Team
+from .account import Account
+from .subscription import Subscription
+from .plan_limits import PlanLimits
+from .account_invite import AccountInvite
 from .tree import Tree
 from .session import Session
 from .attachment import Attachment
@@ -15,6 +19,10 @@ from .audit_log import AuditLog
 __all__ = [
     "User",
     "Team",
+    "Account",
+    "Subscription",
+    "PlanLimits",
+    "AccountInvite",
     "Tree",
     "Session",
     "Attachment",