Show IQDB checks in the metadata modal and run them for visual matches

The modal held a snapshot of the staged upload, so IQDB results that landed
from the background check queue never appeared until it was closed and
reopened — the only hint a check was running was the e621 request history.
It now follows the live uploads query, so candidates, progress and errors
show up in place.

Related gaps fixed along the way:
- files flagged by the local visual-similarity check were skipped by the
  IQDB pass entirely (only 'pending' files were checked), so their modal
  could only ever show 'already in your library'; unresolved files of both
  statuses are now checked, and the check button shows on visual-match
  cards too;
- a check with no candidates posted nothing, leaving 'never checked' and
  'checked, no match' indistinguishable; results are stored even when
  empty and the modal now says which one it is;
- per-file failures surface in the modal instead of being swallowed, the
  modal shows a spinner while the query runs and a check now/re-check
  button, and auto-runs skip files already checked (and videos, since IQDB
  is image-only).

Backend production code unchanged; tests pin the empty-result recording
(18 library tests, full suite 56 green).

backend/apps/library/tests/test_uploads.py

@@ -1,4 +1,4 @@
-"""Staged uploads: complete-set listing and the bulk rating tool."""
+"""Staged uploads: complete-set listing, bulk rating and IQDB recording."""
 
 import base64
 import hashlib
@@ -195,3 +195,88 @@ class BulkResolveTests(TestCase):
         self.assertEqual(len(body["resolved"]), 1)
         self.assertEqual(body["errors"][0]["error"], "already in the library")
         self.assertEqual(MediaItem.objects.count(), 1)
+
+
+class IqdbRecordingTests(TestCase):
+    """The modal needs to tell "checked, no match" from "never checked"."""
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+        cls._tmp = tempfile.mkdtemp(prefix="j621-iqdb-")
+        cls._settings = override_settings(MEDIA_ROOT=cls._tmp)
+        cls._settings.enable()
+
+    @classmethod
+    def tearDownClass(cls):
+        cls._settings.disable()
+        shutil.rmtree(cls._tmp, ignore_errors=True)
+        super().tearDownClass()
+
+    def setUp(self):
+        self.uploader = User.objects.create_user(
+            username="iqdb-uploader", password="iqdb-pass-123456"
+        )
+        self.uploader.role = "uploader"
+        self.uploader.save(update_fields=["role"])
+        self.client = Client()
+        self.client.defaults["HTTP_AUTHORIZATION"] = (
+            f"Token {Token.objects.create(user=self.uploader).key}"
+        )
+
+    def make_temp(self):
+        return TempUpload.objects.create(
+            user=self.uploader,
+            file=SimpleUploadedFile(
+                "checked.png", TINY_PNG, content_type="image/png"
+            ),
+            original_filename="checked.png",
+            md5=hashlib.md5(b"checked").hexdigest(),
+            size=len(TINY_PNG),
+        )
+
+    def test_empty_result_records_the_check_without_a_match(self):
+        temp = self.make_temp()
+        response = jpost(
+            self.client, f"/api/uploads/{temp.id}/iqdb/", {"results": []}
+        )
+        self.assertEqual(response.status_code, 200)
+        temp.refresh_from_db()
+        self.assertEqual(temp.iqdb_data, [])
+        # No candidates must not masquerade as a visual match.
+        self.assertEqual(temp.status, TempUpload.STATUS_PENDING)
+
+    def test_candidates_are_stored_and_flag_a_visual_match(self):
+        temp = self.make_temp()
+        response = jpost(
+            self.client,
+            f"/api/uploads/{temp.id}/iqdb/",
+            {
+                "results": [
+                    {
+                        "post_id": 123,
+                        "score": 91.5,
+                        "preview_url": "https://static1.e621.net/data/preview/ab/cd/x.jpg",
+                        "rating": "q",
+                        "md5": "a" * 32,
+                        "score_total": 12,
+                        "fav_count": 3,
+                        "width": 800,
+                        "height": 600,
+                        "tags_preview": ["canine", "solo"],
+                    }
+                ]
+            },
+        )
+        self.assertEqual(response.status_code, 200)
+        temp.refresh_from_db()
+        self.assertEqual(len(temp.iqdb_data), 1)
+        self.assertEqual(temp.iqdb_data[0]["post_id"], 123)
+        self.assertEqual(temp.status, TempUpload.STATUS_VISUAL_MATCH)
+
+    def test_rejects_a_non_list_payload(self):
+        temp = self.make_temp()
+        response = jpost(
+            self.client, f"/api/uploads/{temp.id}/iqdb/", {"results": "nope"}
+        )
+        self.assertEqual(response.status_code, 400)