batch3 T1 review fixes: Pane discriminated union + index naming

- Restructure Pane as a tagged union over kind so checking kind narrows state
- Rename session_panes_session_idx -> idx_session_panes_session for naming
  consistency with idx_sessions_project, idx_messages_session

apps/server/src/schema.sql

@@ -56,4 +56,4 @@ CREATE TABLE IF NOT EXISTS session_panes (
   created_at   TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp(),
   UNIQUE (session_id, position)
 );
-CREATE INDEX IF NOT EXISTS session_panes_session_idx ON session_panes (session_id);
+CREATE INDEX IF NOT EXISTS idx_session_panes_session ON session_panes (session_id);