fix(boards): replace global cache with React cache() per-request dedup

The unstable_cache approach bypassed throwIfActionForbiddenAsync and
dropped user-specific data (permissions, collapse states). Use React
cache() to deduplicate getInitialBoard within a single request instead,
preserving auth checks and all user data while reducing DB calls from
9 to 3 per board page load.

packages/api/package.json

@@ -10,8 +10,7 @@
     "./server": "./src/server.ts",
     "./websocket": "./src/websocket.ts",
     "./shared": "./src/shared.ts",
-    "./open-api": "./src/open-api.ts",
-    "./router/board": "./src/router/board-exports.ts"
+    "./open-api": "./src/open-api.ts"
   },
   "main": "./index.ts",
   "types": "./index.ts",