fix: correct tree tags subquery in template_trees migration

The INSERT into template_trees incorrectly referenced `tags` as a column
on the `trees` table. Tags are a relationship via the `tree_tag_assignments`
join table — there is no direct column. Migration was failing with:

  UndefinedColumn: column "tags" does not exist ... FROM trees

Fixed by replacing COALESCE(tags, '[]') with a correlated subquery that
aggregates tag names from tree_tag_assignments → tree_tags.

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

backend/alembic/versions/3a40fe11b427_create_global_content_tables.py

@@ -51,16 +51,26 @@ def upgrade() -> None:
     op.create_index('ix_platform_steps_step_type', 'platform_steps', ['step_type'])
 
     # ── Copy is_default=TRUE trees → template_trees ─────────────────────────
+    # Note: trees.tags is a relationship via tree_tags join table — no direct column.
+    # Aggregate tag names via a correlated subquery.
     op.execute("""
         INSERT INTO template_trees
             (id, name, description, category, tree_type, tree_structure,
              tags, is_active, created_at, updated_at, source_tree_id)
         SELECT
-            gen_random_uuid(), name, description, category, tree_type,
-            tree_structure, COALESCE(tags, '[]'::jsonb), is_active,
-            COALESCE(created_at, NOW()), COALESCE(updated_at, NOW()), id
-        FROM trees
-        WHERE is_default = TRUE
+            gen_random_uuid(), t.name, t.description, t.category, t.tree_type,
+            t.tree_structure,
+            COALESCE(
+                (SELECT jsonb_agg(tt.name ORDER BY tt.name)
+                 FROM tree_tag_assignments ta
+                 JOIN tree_tags tt ON tt.id = ta.tag_id
+                 WHERE ta.tree_id = t.id),
+                '[]'::jsonb
+            ),
+            t.is_active,
+            COALESCE(t.created_at, NOW()), COALESCE(t.updated_at, NOW()), t.id
+        FROM trees t
+        WHERE t.is_default = TRUE
     """)
 
     # ── Copy visibility='public' steps → platform_steps ─────────────────────