feat: Phase 1 Group 9 — enforce NOT NULL on all account_id columns

All previously-nullable account_id columns are now NOT NULL.
tree_embeddings and feedback backfilled before constraint applied.
Global content assigned to platform sentinel account (00000000-...-0001)
in preceding migration.

Tables updated: users, trees, tree_categories, tree_tags,
step_categories, step_library, tree_embeddings, feedback

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

backend/alembic/versions/174f442795b7_set_not_null_account_id_phase1.py

@@ -0,0 +1,86 @@
+"""set NOT NULL on all previously-nullable account_id columns
+
+Revision ID: 174f442795b7
+Revises: 3a40fe11b427
+Create Date: 2026-04-09 00:00:00.000000
+
+All tables in this migration had account_id set to nullable previously.
+Task 9 (create_global_content_tables) cleared all NULL rows.
+This migration enforces the NOT NULL constraint.
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '174f442795b7'
+down_revision: Union[str, None] = '3a40fe11b427'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # tree_embeddings: backfill from trees (must happen before SET NOT NULL)
+    op.execute("""
+        UPDATE tree_embeddings te
+        SET account_id = t.account_id
+        FROM trees t
+        WHERE te.tree_id = t.id
+          AND te.account_id IS NULL
+    """)
+
+    # feedback: backfill from users
+    op.execute("""
+        UPDATE feedback f
+        SET account_id = u.account_id
+        FROM users u
+        WHERE f.user_id = u.id
+          AND f.account_id IS NULL
+    """)
+
+    # Verify ALL tables before touching any SET NOT NULL
+    tables_with_account_id = [
+        'users', 'trees', 'tree_categories', 'tree_tags',
+        'step_categories', 'step_library', 'tree_embeddings', 'feedback',
+    ]
+    for table in tables_with_account_id:
+        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}. "
+                "Run Task 9 (create_global_content_tables) first, or "
+                "manually backfill/delete orphaned rows."
+            )
+
+    # SET NOT NULL on all
+    for table in tables_with_account_id:
+        op.alter_column(table, 'account_id', nullable=False)
+
+    # Create indexes where they don't already exist
+    new_indexes = [
+        ('tree_embeddings', 'ix_tree_embeddings_account_id'),
+        ('feedback', 'ix_feedback_account_id'),
+    ]
+    for table, index_name in new_indexes:
+        result = op.get_bind().execute(sa.text(
+            f"SELECT 1 FROM pg_indexes WHERE tablename='{table}' AND indexname='{index_name}'"
+        ))
+        if not result.fetchone():
+            op.create_index(index_name, table, ['account_id'])
+
+
+def downgrade() -> None:
+    # Revert to nullable
+    for table in ('users', 'trees', 'tree_categories', 'tree_tags',
+                  'step_categories', 'step_library', 'tree_embeddings', 'feedback'):
+        op.alter_column(table, 'account_id', nullable=True)
+    for table, index_name in (
+        ('tree_embeddings', 'ix_tree_embeddings_account_id'),
+        ('feedback', 'ix_feedback_account_id'),
+    ):
+        try:
+            op.drop_index(index_name, table_name=table)
+        except Exception:
+            pass