feat(boards): cache named board data with tag-based invalidation

Named boards are now served from Next.js unstable_cache (1h TTL,
tag `board-{name}`). A new `getSharedBoardWithWhereAsync` helper
fetches the board without per-user data so the result is safe to
share across requests.

Cache is busted via `invalidateBoardCacheAsync` (revalidateTag +
revalidatePath) from board save, layout save, and partial settings
mutations.

packages/api/package.json

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