batch3 T1: session_panes schema + Pane/UserStreamFrame types + sidebar project_id

Adds the session_panes table, Pane/PaneState/PaneCreate/PaneUpdate types,
UserStreamFrame discriminated union, and extends SidebarSession with
project_id (also added to the sidebar SELECT).

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

apps/server/src/schema.sql

@@ -46,3 +46,14 @@ CREATE TABLE IF NOT EXISTS settings (
 );
 
 INSERT INTO settings (key, value) VALUES ('default_model', '"qwen3.6-35b-a3b-mxfp4"') ON CONFLICT (key) DO NOTHING;
+
+CREATE TABLE IF NOT EXISTS session_panes (
+  id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  session_id   UUID NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
+  position     INTEGER NOT NULL,
+  kind         TEXT NOT NULL CHECK (kind IN ('chat', 'file_browser')),
+  state        JSONB NOT NULL DEFAULT '{}',
+  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);