Fix uploads stuck pending due to page navigation killing sync

The sync engine was adding per-photo backoff delays (2-60s) inside
the processing loop, during which the user would navigate to another
page (browser -> queue -> browser), killing the sync mid-wait. The
upload POST never fired because the delay ran out the clock.

Changes:
- Remove per-photo backoff delay from processQueue loop — uploads
  start immediately on page load without blocking
- Move retry scheduling to after the full queue pass: if any uploads
  failed, schedule a new triggerSync() after 15s instead of blocking
  inline
- Keep the duplicate check on retries (fast, catches interrupted
  uploads that actually succeeded server-side)
- uploadPhoto now returns true/false so processQueue can track
  failures

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

app/static/js/sync.js

@@ -10,8 +10,7 @@ const Sync = {
     MAX_RETRIES: 15,
     VERIFY_DELAY_MS: 800,
     UPLOAD_TIMEOUT_MS: 120000,
-    // Backoff delays in ms per retry attempt
-    BACKOFF_DELAYS: [0, 2000, 5000, 10000, 15000, 30000, 30000, 60000, 60000, 60000, 60000, 60000, 60000, 60000, 60000],
+    _retryTimerId: null,
 
     init(username) {
         this.currentUsername = username;
@@ -34,6 +33,10 @@ const Sync = {
             console.log('[SYNC] Network offline');
             this.isOnline = false;
             this.isSyncing = false;
+            if (this._retryTimerId) {
+                clearTimeout(this._retryTimerId);
+                this._retryTimerId = null;
+            }
         });
     },
 
@@ -63,14 +66,26 @@ const Sync = {
         console.log('[SYNC] Starting sync...');
         this.isSyncing = true;
 
+        let hadFailures = false;
         try {
-            await this.processQueue();
+            hadFailures = await this.processQueue();
         } catch (error) {
             console.error('[SYNC] Error:', error);
+            hadFailures = true;
         } finally {
             this.isSyncing = false;
             this._setUploading(false);
         }
+
+        // If there were failures, schedule a retry cycle after a delay
+        // so it doesn't block the current page load
+        if (hadFailures && this.isOnline) {
+            console.log('[SYNC] Scheduling retry in 15s...');
+            this._retryTimerId = setTimeout(() => {
+                this._retryTimerId = null;
+                this.triggerSync();
+            }, 15000);
+        }
     },
 
     async processQueue() {
@@ -82,9 +97,11 @@ const Sync = {
         console.log('[SYNC] Found', pendingPhotos.length, 'photos to process');
 
         if (pendingPhotos.length === 0) {
-            return;
+            return false;
         }
 
+        let hadFailures = false;
+
         for (const photo of pendingPhotos) {
             if (!this.isOnline) {
                 console.log('[SYNC] Lost connection, stopping');
@@ -99,21 +116,13 @@ const Sync = {
                 continue;
             }
 
-            // Backoff delay before retry attempts
-            if (retryCount > 0) {
-                const backoff = this.BACKOFF_DELAYS[Math.min(retryCount, this.BACKOFF_DELAYS.length - 1)];
-                console.log('[SYNC] Waiting', backoff + 'ms before retry', retryCount + 1);
-                await this.delay(backoff);
-
-                // Re-check connectivity after waiting
-                if (!this.isOnline) {
-                    console.log('[SYNC] Lost connection during backoff, stopping');
-                    break;
-                }
-            }
-
-            await this.uploadPhoto(photo);
+            // No delay — upload immediately. Retries are handled by
+            // scheduling a new triggerSync() after the full queue pass.
+            const success = await this.uploadPhoto(photo);
+            if (!success) hadFailures = true;
         }
+
+        return hadFailures;
     },
 
     async uploadPhoto(photo) {
@@ -134,7 +143,8 @@ const Sync = {
             // Prevent page navigation during upload
             this._setUploading(true);
 
-            // Check for duplicates on retries (skip on first attempt to reduce latency)
+            // On retries, first check if a previous attempt actually succeeded
+            // (the upload may have completed but the verify/status update was interrupted)
             if (retryCount > 0) {
                 const fullPath = photo.targetPath.replace(/\/$/, '') + '/' + photo.filename;
                 const alreadyExists = await this.checkFileExists(fullPath);
@@ -148,7 +158,7 @@ const Sync = {
                     });
                     await this.pruneHistory();
                     this._setUploading(false);
-                    return;
+                    return true;
                 }
             }
 
@@ -227,6 +237,7 @@ const Sync = {
 
             // Clear navigation guard after successful upload
             this._setUploading(false);
+            return true;
 
         } catch (error) {
             this._setUploading(false);
@@ -237,6 +248,7 @@ const Sync = {
                 lastError: error.message,
                 error: error.message
             });
+            return false;
         }
     },
 
@@ -264,7 +276,6 @@ const Sync = {
             const result = await response.json();
             return result.exists === true;
         } catch (e) {
-            // If we can't check, assume it doesn't exist and proceed with upload
             return false;
         }
     },