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/019_migrate_team_fks_to_account.py

@@ -0,0 +1,56 @@
+"""add account_id to content tables and backfill from _team_account_mapping
+
+Revision ID: 019
+Revises: 018
+Create Date: 2026-02-07
+
+Uses the _team_account_mapping table from migration 018 for deterministic
+FK backfill instead of non-deterministic LIMIT 1 subqueries.
+"""
+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 = '019'
+down_revision: Union[str, None] = '018'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+# Tables that have team_id and need account_id
+CONTENT_TABLES = ['trees', 'step_library', 'tree_categories', 'tree_tags', 'step_categories']
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+
+    for table in CONTENT_TABLES:
+        # Add account_id column
+        op.add_column(table, sa.Column('account_id', UUID(as_uuid=True), nullable=True))
+        op.create_index(f'ix_{table}_account_id', table, ['account_id'])
+
+        # Backfill from mapping table (deterministic)
+        conn.execute(sa.text(f"""
+            UPDATE {table} SET account_id = m.account_id
+            FROM _team_account_mapping m
+            WHERE {table}.team_id = m.team_id
+        """))
+
+        # Validate: no rows with team_id but missing account_id
+        orphaned = conn.execute(sa.text(f"""
+            SELECT COUNT(*) FROM {table}
+            WHERE team_id IS NOT NULL AND account_id IS NULL
+        """)).scalar()
+        if orphaned > 0:
+            raise RuntimeError(
+                f"Migration 019 failed: {table} has {orphaned} rows with team_id but no account_id"
+            )
+
+
+def downgrade() -> None:
+    for table in reversed(CONTENT_TABLES):
+        op.drop_index(f'ix_{table}_account_id', table_name=table)
+        op.drop_column(table, 'account_id')