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;
   }
 
@@ -544,7 +545,10 @@ class UploadManager extends EventEmitter {
     });
   }
 
-  async _waitForInterval(hoster, intervalMs, signal) {
+  _waitForInterval(hoster, intervalMs, signal) {
+    // Serialize interval waits per hoster so concurrent jobs queue up properly
+    const prev = this.intervalLocks[hoster] || Promise.resolve();
+    const next = prev.then(async () => {
       const now = Date.now();
       const last = this.lastStartTime[hoster] || 0;
       const elapsed = now - last;
@@ -552,6 +556,9 @@ class UploadManager extends EventEmitter {
         await this._sleep(intervalMs - elapsed, signal);
       }
       this.lastStartTime[hoster] = Date.now();
+    });
+    this.intervalLocks[hoster] = next.catch(() => {});
+    return next;
   }
 
   cancelJobs(jobIds) {