feat: Phase 1 Group 1 — add account_id to core session tables

Migration sequence: add nullable → backfill via user_id/ai_session chain
→ verify zero NULLs → SET NOT NULL → CREATE INDEX.

Tables: sessions, attachments, session_supporting_data,
        session_resolution_outputs

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

backend/alembic/versions/cc214c63aa30_add_account_id_core_sessions.py

@@ -0,0 +1,95 @@
+"""add account_id to core session tables
+
+Revision ID: cc214c63aa30
+Revises: b8d2f4a6c091
+Create Date: 2026-04-09 00:00:00.000000
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = 'cc214c63aa30'
+down_revision: Union[str, None] = 'b8d2f4a6c091'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ── Step 1: ADD COLUMN (nullable) ────────────────────────────────────────
+    for table in ('sessions', 'attachments', 'session_supporting_data',
+                  'session_resolution_outputs'):
+        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',
+        )
+
+    # ── Step 2: BACKFILL ─────────────────────────────────────────────────────
+    # sessions: direct join to users
+    op.execute("""
+        UPDATE sessions s
+        SET account_id = u.account_id
+        FROM users u
+        WHERE s.user_id = u.id
+          AND s.account_id IS NULL
+    """)
+
+    # attachments: chain through sessions (now backfilled above)
+    op.execute("""
+        UPDATE attachments a
+        SET account_id = s.account_id
+        FROM sessions s
+        WHERE a.session_id = s.id
+          AND a.account_id IS NULL
+    """)
+
+    # session_supporting_data: same chain
+    op.execute("""
+        UPDATE session_supporting_data sd
+        SET account_id = s.account_id
+        FROM sessions s
+        WHERE sd.session_id = s.id
+          AND sd.account_id IS NULL
+    """)
+
+    # session_resolution_outputs: FK is to ai_sessions, not sessions
+    op.execute("""
+        UPDATE session_resolution_outputs sro
+        SET account_id = ai.account_id
+        FROM ai_sessions ai
+        WHERE sro.session_id = ai.id
+          AND sro.account_id IS NULL
+    """)
+
+    # ── Step 3: VERIFY zero NULLs — raises if any remain ────────────────────
+    for table in ('sessions', 'attachments', 'session_supporting_data',
+                  'session_resolution_outputs'):
+        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 remain in {table}. "
+                f"Fix the backfill before re-running."
+            )
+
+    # ── Step 4: SET NOT NULL ─────────────────────────────────────────────────
+    for table in ('sessions', 'attachments', 'session_supporting_data',
+                  'session_resolution_outputs'):
+        op.alter_column(table, 'account_id', nullable=False)
+
+    # ── Step 5: CREATE INDEX ─────────────────────────────────────────────────
+    for table in ('sessions', 'attachments', 'session_supporting_data',
+                  'session_resolution_outputs'):
+        op.create_index(f'ix_{table}_account_id', table, ['account_id'])
+
+
+def downgrade() -> None:
+    for table in ('sessions', 'attachments', 'session_supporting_data',
+                  'session_resolution_outputs'):
+        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')