Add Planner session management with Yjs WebSocket (#1)

- Yjs WebSocket server at /sync/:sessionId for real-time collaboration
- Session creation API (POST /api/sessions) with optional GPX initialization
- Session join page with Planner layout (header, map area, sidebar)
- In-memory session store (PostgreSQL persistence deferred to infra setup)
- Custom production server entry with WebSocket upgrade handling
- Vite path aliases (~/) for clean imports
- Fix tsconfig for apps: noEmit, exclude build dir

Tasks completed: 4.1, 4.3, 4.4, 4.5
Deferred: 4.2 (PostgreSQL), 4.6-4.8 (need client-side Yjs from Group 6)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/planner/tsconfig.json

@@ -5,9 +5,13 @@
     "**/*.tsx",
     ".react-router/types/**/*"
   ],
+  "exclude": ["build", "node_modules", "server.ts"],
   "compilerOptions": {
     "rootDirs": [".", "./.react-router/types"],
-    "outDir": "build",
-    "types": ["vite/client"]
+    "noEmit": true,
+    "types": ["vite/client"],
+    "paths": {
+      "~/*": ["./app/*"]
+    }
   }
 }