batch3 T2: panes CRUD route + default chat pane on session POST

Adds /api/sessions/:id/panes (GET, POST), /api/panes/:id (PATCH, DELETE)
with transactional position-shift logic (negate-and-restore pattern to
avoid UNIQUE collisions). Max 5 panes per session enforced.

Sessions.POST now creates the session and a default Chat pane at position
0 atomically via sql.begin.

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

apps/server/src/index.ts

@@ -13,6 +13,7 @@ import { registerMessageRoutes } from './routes/messages.js';
 import { registerSidebarRoutes } from './routes/sidebar.js';
 import { registerWebSocket } from './routes/ws.js';
 import { registerModelRoutes } from './routes/models.js';
+import { registerPaneRoutes } from './routes/panes.js';
 import { createInferenceRunner } from './services/inference.js';
 import { createBroker } from './services/broker.js';
 
@@ -41,6 +42,7 @@ async function main() {
   registerSettingsRoutes(app, sql);
   registerModelRoutes(app, config);
   registerSidebarRoutes(app, sql);
+  registerPaneRoutes(app, sql);
 
   const broker = createBroker();
   const inference = createInferenceRunner({