fix(import): scan_directory skips paths with existing non-failed ImportTask

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/tasks/scan.py

@@ -65,16 +65,34 @@ def scan_directory(self, triggered_by: str = "manual",
         session.flush()
         batch_id = batch.id
 
+        # Skip-set: any source_path that already has a non-failed ImportTask
+        # row. Re-running scan_directory must not re-enqueue files the
+        # importer has already handled (or is currently handling); doing so
+        # creates duplicate work and inflates the queue. Failed prior tasks
+        # are eligible for retry.
+        non_failed_existing = set(session.execute(
+            select(ImportTask.source_path).where(
+                ImportTask.status.in_(
+                    ["pending", "queued", "processing", "complete", "skipped"]
+                ),
+            )
+        ).scalars().all())
+
         # Walk and enumerate.
         files_seen = 0
+        files_skipped_existing = 0
         for entry in _iter_import_files(import_root):
+            entry_str = str(entry)
+            if entry_str in non_failed_existing:
+                files_skipped_existing += 1
+                continue
             try:
                 size = entry.stat().st_size
             except OSError:
                 size = None
             task = ImportTask(
                 batch_id=batch_id,
-                source_path=str(entry),
+                source_path=entry_str,
                 task_type="media",
                 status="pending",
                 size_bytes=size,