fix: handle non-default, no-team trees in global content migration

Migration 019 only backfills trees with team_id IS NOT NULL.
Migration 3a40fe11b427 only covered is_default=TRUE trees.
Trees with team_id=NULL and is_default=FALSE (e.g. inactive test trees,
pre-team-system content) fell through both passes and triggered the NULL
guard.

Add two new UPDATE steps after the is_default pass:
1. Assign remaining trees to their author's account (if author has one)
2. Final fallback to PLATFORM_ACCOUNT_ID for any still-NULL rows

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

backend/alembic/versions/3a40fe11b427_create_global_content_tables.py

@@ -105,6 +105,26 @@ def upgrade() -> None:
           AND account_id IS NULL
     """)
 
+    # ── Assign remaining trees to their author's account ─────────────────────
+    # Handles trees with no team_id that aren't is_default (e.g. inactive test
+    # trees, trees created before the team system existed).
+    op.execute("""
+        UPDATE trees
+        SET account_id = u.account_id
+        FROM users u
+        WHERE trees.author_id = u.id
+          AND trees.account_id IS NULL
+          AND u.account_id IS NOT NULL
+    """)
+
+    # ── Final fallback: any still-NULL trees go to platform account ───────────
+    # Covers trees whose author has no account (seeded content, system rows).
+    op.execute("""
+        UPDATE trees
+        SET account_id = '00000000-0000-0000-0000-000000000001'
+        WHERE account_id IS NULL
+    """)
+
     # ── Assign global categories/tags/steps to platform account ─────────────
     op.execute("""
         UPDATE tree_categories