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/tests/test_phase1_migrations.py

@@ -7,7 +7,7 @@ real PostgreSQL test DB (same as all other integration tests).
 import pytest
 import uuid
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select
+from sqlalchemy import select, text
 
 from app.models.account import Account
 from app.models.user import User
@@ -506,3 +506,40 @@ async def test_platform_steps_table_exists_and_has_no_account_id(test_db: AsyncS
     columns = {row[0] for row in result.fetchall()}
     assert 'id' in columns, "platform_steps.id must exist"
     assert 'account_id' not in columns, "platform_steps must not have account_id (global content)"
+
+
+# ── Group 9: SET NOT NULL on existing nullable columns ────────────────────────
+
+@pytest.mark.asyncio
+async def test_tree_account_id_is_not_null(test_db: AsyncSession):
+    """trees.account_id must be NOT NULL after Phase 1 — enforced at DB level."""
+    from sqlalchemy.exc import IntegrityError
+    with pytest.raises(IntegrityError):
+        test_db.add(Tree(
+            name="Bad tree",
+            # account_id intentionally omitted
+            author_id=None,
+            visibility="private",
+            tree_type="troubleshooting",
+            tree_structure={},
+            is_active=True,
+            status="draft",
+        ))
+        await test_db.flush()
+
+
+@pytest.mark.asyncio
+async def test_user_account_id_is_not_null(test_db: AsyncSession):
+    """users.account_id must be NOT NULL after Phase 1."""
+    from sqlalchemy.exc import IntegrityError
+    with pytest.raises(IntegrityError):
+        test_db.add(User(
+            email=f"orphan-{uuid.uuid4().hex[:6]}@example.com",
+            name="Orphan",
+            password_hash=get_password_hash("x"),
+            is_active=True,
+            role="engineer",
+            account_role="engineer",
+            # account_id intentionally omitted
+        ))
+        await test_db.flush()