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/alembic/versions/017_add_account_id_to_users.py

@@ -0,0 +1,31 @@
+"""add account_id and account_role columns to users
+
+Revision ID: 017
+Revises: 016
+Create Date: 2026-02-07
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID
+
+
+# revision identifiers, used by Alembic.
+revision: str = '017'
+down_revision: Union[str, None] = '016'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('users', sa.Column('account_id', UUID(as_uuid=True), nullable=True))
+    op.add_column('users', sa.Column('account_role', sa.String(50), nullable=True))
+    op.create_index('ix_users_account_id', 'users', ['account_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_users_account_id', table_name='users')
+    op.drop_column('users', 'account_role')
+    op.drop_column('users', 'account_id')