batch3 final: backfill default chat pane for pre-batch3 sessions

Sessions created before Batch 3 have no rows in session_panes, so the
Workspace renders "No panes" on first open. Idempotent INSERT inserts
a default chat pane at position 0 for any session without one. NOT
EXISTS guard makes the statement a no-op after the first run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/schema.sql

@@ -57,3 +57,12 @@ CREATE TABLE IF NOT EXISTS session_panes (
   UNIQUE (session_id, position)
 );
 CREATE INDEX IF NOT EXISTS idx_session_panes_session ON session_panes (session_id);
+
+-- Backfill: ensure every session has at least one pane (default Chat).
+-- Idempotent: skipped on subsequent runs because session_panes rows already exist.
+INSERT INTO session_panes (session_id, position, kind, state)
+SELECT s.id, 0, 'chat', '{}'::jsonb
+FROM sessions s
+WHERE NOT EXISTS (
+  SELECT 1 FROM session_panes p WHERE p.session_id = s.id
+);