feat(kb): requeue-failed endpoint

Moves every object under failed/<kind>/ back to inbox/<kind>/ for retry
after a transient upstream failure (e.g. Voyage 401 before a key rotation).

Refs Task Master #2

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

api/services/kb/s3.py

@@ -95,6 +95,25 @@ def mark_processed(src_key: str, filename: str, kind: str) -> str:
     return dst
 
 
+def list_failed() -> list[dict]:
+    s3 = _client()
+    bucket = _bucket()
+    items: list[dict] = []
+    for kind in _KINDS:
+        prefix = f"failed/{kind}/"
+        paginator = s3.get_paginator("list_objects_v2")
+        for page in paginator.paginate(Bucket=bucket, Prefix=prefix):
+            for obj in page.get("Contents", []) or []:
+                key = obj["Key"]
+                if key.endswith("/"):
+                    continue
+                filename = key[len(prefix):]
+                if not filename:
+                    continue
+                items.append({"key": key, "kind": kind, "filename": filename, "size": obj["Size"]})
+    return items
+
+
 def mark_failed(src_key: str, filename: str, kind: str, reason: str) -> str:
     s3 = _client()
     bucket = _bucket()