feat: Phase 1 Group 8 — add account_id to target_lists (keep team_id)

Zero rows in production — this is a schema-only migration in practice.
team_id kept for app code compatibility. Drop deferred to later cleanup.
Backfill: team_id → team admin user → account_id; fallback: created_by.

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

backend/alembic/versions/2c6aabd89bc6_add_account_id_target_lists.py

@@ -0,0 +1,62 @@
+"""add account_id to target_lists (keep team_id)
+
+Revision ID: 2c6aabd89bc6
+Revises: 78fc200abac1
+Create Date: 2026-04-09 00:00:00.000000
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '2c6aabd89bc6'
+down_revision: Union[str, None] = '78fc200abac1'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('target_lists', sa.Column('account_id', sa.UUID(), nullable=True))
+    op.create_foreign_key(
+        'fk_target_lists_account_id', 'target_lists', 'accounts',
+        ['account_id'], ['id'], ondelete='CASCADE',
+    )
+
+    # Primary: team_id → team admin user → account_id
+    op.execute("""
+        UPDATE target_lists tl
+        SET account_id = u.account_id
+        FROM users u
+        WHERE u.team_id = tl.team_id
+          AND u.is_team_admin = TRUE
+          AND u.account_id IS NOT NULL
+          AND tl.account_id IS NULL
+    """)
+
+    # Fallback: created_by → users.account_id
+    op.execute("""
+        UPDATE target_lists tl
+        SET account_id = u.account_id
+        FROM users u
+        WHERE tl.created_by = u.id
+          AND u.account_id IS NOT NULL
+          AND tl.account_id IS NULL
+    """)
+
+    result = op.get_bind().execute(
+        sa.text("SELECT COUNT(*) FROM target_lists WHERE account_id IS NULL")
+    )
+    count = result.scalar()
+    if count > 0:
+        raise RuntimeError(
+            f"ROLLBACK: {count} target_lists rows have NULL account_id. "
+            "No team admin found for these teams. Resolve before re-running."
+        )
+
+    op.alter_column('target_lists', 'account_id', nullable=False)
+    op.create_index('ix_target_lists_account_id', 'target_lists', ['account_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_target_lists_account_id', table_name='target_lists')
+    op.drop_constraint('fk_target_lists_account_id', 'target_lists', type_='foreignkey')
+    op.drop_column('target_lists', 'account_id')