fix: serialize interval waits so uploads stagger correctly

Previously all jobs read the same lastStartTime simultaneously,
causing them all to start at once. Now uses a per-hoster promise
chain to ensure each job waits its turn.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/upload-manager.js

@@ -35,6 +35,7 @@ class UploadManager extends EventEmitter {
     this.cancelledJobIds = new Set();
     this.sessionBytes = 0;
     this.lastStartTime = {}; // hoster -> timestamp of last upload start
+    this.intervalLocks = {}; // hoster -> Promise chain for serialized interval waits
     this.globalThrottle = null;
   }
 
@@ -544,14 +545,20 @@ class UploadManager extends EventEmitter {
     });
   }
 
-  async _waitForInterval(hoster, intervalMs, signal) {
+  _waitForInterval(hoster, intervalMs, signal) {
-    const now = Date.now();
+    // Serialize interval waits per hoster so concurrent jobs queue up properly
-    const last = this.lastStartTime[hoster] || 0;
+    const prev = this.intervalLocks[hoster] || Promise.resolve();
-    const elapsed = now - last;
+    const next = prev.then(async () => {
-    if (elapsed < intervalMs) {
+      const now = Date.now();
-      await this._sleep(intervalMs - elapsed, signal);
+      const last = this.lastStartTime[hoster] || 0;
-    }
+      const elapsed = now - last;
-    this.lastStartTime[hoster] = Date.now();
+      if (elapsed < intervalMs) {
+        await this._sleep(intervalMs - elapsed, signal);
+      }
+      this.lastStartTime[hoster] = Date.now();
+    });
+    this.intervalLocks[hoster] = next.catch(() => {});
+    return next;
   }
 
   cancelJobs(jobIds) {