feat: Phase 1 Group 2 — add account_id to AI branching tables

Tables: session_branches, session_handoffs, fork_points,
        ai_session_steps, ai_suggestions
Backfill: session_id → ai_sessions.account_id (all except
ai_suggestions which uses user_id → users.account_id)

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

backend/alembic/versions/478c159e5654_add_account_id_ai_branching.py

@@ -0,0 +1,77 @@
+"""add account_id to AI branching tables
+
+Revision ID: 478c159e5654
+Revises: cc214c63aa30
+Create Date: 2026-04-09 00:00:00.000000
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '478c159e5654'
+down_revision: Union[str, None] = 'cc214c63aa30'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    ai_tables = ('session_branches', 'session_handoffs', 'fork_points', 'ai_session_steps')
+
+    # Step 1: ADD COLUMN (nullable)
+    for table in ai_tables:
+        op.add_column(table, sa.Column('account_id', sa.UUID(), nullable=True))
+        op.create_foreign_key(
+            f'fk_{table}_account_id', table, 'accounts',
+            ['account_id'], ['id'], ondelete='CASCADE',
+        )
+
+    op.add_column('ai_suggestions', sa.Column('account_id', sa.UUID(), nullable=True))
+    op.create_foreign_key(
+        'fk_ai_suggestions_account_id', 'ai_suggestions', 'accounts',
+        ['account_id'], ['id'], ondelete='CASCADE',
+    )
+
+    # Step 2: BACKFILL
+    for table in ai_tables:
+        op.execute(f"""
+            UPDATE {table} t
+            SET account_id = ai.account_id
+            FROM ai_sessions ai
+            WHERE t.session_id = ai.id
+              AND t.account_id IS NULL
+        """)
+
+    op.execute("""
+        UPDATE ai_suggestions s
+        SET account_id = u.account_id
+        FROM users u
+        WHERE s.user_id = u.id
+          AND s.account_id IS NULL
+    """)
+
+    # Step 3: VERIFY zero NULLs
+    for table in ai_tables + ('ai_suggestions',):
+        result = op.get_bind().execute(
+            sa.text(f"SELECT COUNT(*) FROM {table} WHERE account_id IS NULL")
+        )
+        count = result.scalar()
+        if count > 0:
+            raise RuntimeError(
+                f"ROLLBACK: {count} NULL account_id rows in {table}."
+            )
+
+    # Step 4: SET NOT NULL
+    for table in ai_tables + ('ai_suggestions',):
+        op.alter_column(table, 'account_id', nullable=False)
+
+    # Step 5: CREATE INDEX
+    for table in ai_tables + ('ai_suggestions',):
+        op.create_index(f'ix_{table}_account_id', table, ['account_id'])
+
+
+def downgrade() -> None:
+    for table in ('session_branches', 'session_handoffs', 'fork_points',
+                  'ai_session_steps', 'ai_suggestions'):
+        op.drop_index(f'ix_{table}_account_id', table_name=table)
+        op.drop_constraint(f'fk_{table}_account_id', table, type_='foreignkey')
+        op.drop_column(table, 'account_id')