feat(core-api): harden signed urls and llm rate limits

backend/core-api/src/services/llm-rate-limit.js

@@ -0,0 +1,41 @@
+const counters = new Map();
+
+function currentWindowKey(now = Date.now()) {
+  return Math.floor(now / 60000);
+}
+
+function perMinuteLimit() {
+  return Number.parseInt(process.env.LLM_RATE_LIMIT_PER_MINUTE || '20', 10);
+}
+
+export function checkLlmRateLimit({ uid, now = Date.now() }) {
+  const limit = perMinuteLimit();
+  if (!Number.isFinite(limit) || limit <= 0) {
+    return { allowed: true, remaining: null, retryAfterSeconds: 0 };
+  }
+
+  const windowKey = currentWindowKey(now);
+  const record = counters.get(uid);
+
+  if (!record || record.windowKey !== windowKey) {
+    counters.set(uid, { windowKey, count: 1 });
+    return { allowed: true, remaining: limit - 1, retryAfterSeconds: 0 };
+  }
+
+  if (record.count >= limit) {
+    const retryAfterSeconds = (windowKey + 1) * 60 - Math.floor(now / 1000);
+    return {
+      allowed: false,
+      remaining: 0,
+      retryAfterSeconds: Math.max(1, retryAfterSeconds),
+    };
+  }
+
+  record.count += 1;
+  counters.set(uid, record);
+  return { allowed: true, remaining: limit - record.count, retryAfterSeconds: 0 };
+}
+
+export function __resetLlmRateLimitForTests() {
+  counters.clear();
+}

backend/core-api/src/services/storage.js

@@ -3,7 +3,7 @@ import { AppError } from '../lib/errors.js';
 
 const storage = new Storage();
 
-function parseGsUri(fileUri) {
+export function parseGsUri(fileUri) {
   if (!fileUri.startsWith('gs://')) {
     throw new AppError('VALIDATION_ERROR', 'fileUri must start with gs://', 400);
   }
@@ -27,6 +27,20 @@ function allowedBuckets() {
   ]);
 }
 
+export function validateFileUriAccess({ fileUri, actorUid }) {
+  const { bucket, path } = parseGsUri(fileUri);
+  if (!allowedBuckets().has(bucket)) {
+    throw new AppError('FORBIDDEN', `Bucket not allowed for signing: ${bucket}`, 403);
+  }
+
+  const ownedPrefix = `uploads/${actorUid}/`;
+  if (!path.startsWith(ownedPrefix)) {
+    throw new AppError('FORBIDDEN', 'Cannot sign URL for another user path', 403);
+  }
+
+  return { bucket, path };
+}
+
 export async function uploadToGcs({ bucket, objectPath, contentType, buffer }) {
   const file = storage.bucket(bucket).file(objectPath);
   await file.save(buffer, {
@@ -38,13 +52,14 @@ export async function uploadToGcs({ bucket, objectPath, contentType, buffer }) {
   });
 }
 
-export async function generateReadSignedUrl({ fileUri, expiresInSeconds }) {
-  const { bucket, path } = parseGsUri(fileUri);
-  if (!allowedBuckets().has(bucket)) {
-    throw new AppError('FORBIDDEN', `Bucket not allowed for signing: ${bucket}`, 403);
+export async function generateReadSignedUrl({ fileUri, actorUid, expiresInSeconds }) {
+  const { bucket, path } = validateFileUriAccess({ fileUri, actorUid });
+  const file = storage.bucket(bucket).file(path);
+  const [exists] = await file.exists();
+  if (!exists) {
+    throw new AppError('NOT_FOUND', 'File not found for signed URL', 404, { fileUri });
   }
 
-  const file = storage.bucket(bucket).file(path);
   const expiresAtMs = Date.now() + expiresInSeconds * 1000;
   const [signedUrl] = await file.getSignedUrl({
     version: 'v4',