Wait for all uploads, then batch MD5 -> visual -> IQDB with bulk links

Batching must not start while files are still being uploaded, and the MD5
phase must move a whole chunk at once instead of one resolve per file:

- the upload queue drains completely first (failed uploads included) before
  any matching starts;
- phase 1 asks e621 for every md5 (75 per posts.json request), builds the
  md5 -> post map from the response, and sends the matches to the new
  POST /api/uploads/link-bulk/ action, so a whole 75-file chunk moves into
  Indexed in a single board update;
- link-bulk indexes the staged file directly when the post's MD5 matches
  (identical bytes), so there is no per-file download round trip;
- phase 2 runs local visual similarity for whatever stayed pending, phase 3
  the IQDB queue.

Verified end to end with real e621 files: one md5 query for the batch, one
link-bulk call, both matching files flipped to Indexed together, then the
visual and IQDB phases. 23 library tests green (link-bulk, visual phase,
deferred visual matching).

backend/apps/library/tests/test_uploads.py

@@ -132,13 +132,17 @@ class StagedUploadWorkflowTests(TestCase):
         )
         return client
 
-    def make_temp(self, user, label, status=TempUpload.STATUS_PENDING):
+    def make_temp(
+        self, user, label, status=TempUpload.STATUS_PENDING, payload=None
+    ):
         return TempUpload.objects.create(
             user=user,
-            file=SimpleUploadedFile(f"{label}.png", TINY_PNG, content_type="image/png"),
+            file=SimpleUploadedFile(
+                f"{label}.png", payload or TINY_PNG, content_type="image/png"
+            ),
             original_filename=f"{label}.png",
             md5=hashlib.md5(label.encode()).hexdigest(),
-            size=len(TINY_PNG),
+            size=len(payload or TINY_PNG),
             status=status,
         )
 
@@ -261,6 +265,57 @@ class StagedUploadWorkflowTests(TestCase):
         response = client.post(f"/api/uploads/{temp.id}/visual-match/")
         self.assertEqual(response.status_code, 400)
 
+    def test_link_bulk_moves_a_batch_with_one_call(self):
+        client = self.api_client(self.uploader)
+        first = self.make_temp(self.uploader, "bulk-link-1")
+        second = self.make_temp(
+            self.uploader, "bulk-link-2", payload=similar_png((10, 200, 30))
+        )
+        response = jpost(
+            client,
+            "/api/uploads/link-bulk/",
+            {
+                "links": [
+                    {
+                        "temp_id": str(first.id),
+                        "post": {
+                            "id": 900001,
+                            "rating": "s",
+                            "file": {
+                                "md5": first.md5,
+                                # Identical MD5: the staged file is indexed
+                                # without downloading the URL.
+                                "url": "https://static1.e621.net/data/fake-1.png",
+                            },
+                        },
+                    },
+                    {
+                        "temp_id": str(second.id),
+                        "post": {
+                            "id": 900002,
+                            "rating": "q",
+                            "file": {
+                                "md5": second.md5,
+                                "url": "https://static1.e621.net/data/fake-2.png",
+                            },
+                        },
+                    },
+                ]
+            },
+        )
+        self.assertEqual(response.status_code, 200)
+        body = response.json()
+        self.assertEqual(body["errors"], [])
+        self.assertEqual(len(body["updated"]), 2)
+        for temp, post_id in ((first, 900001), (second, 900002)):
+            temp.refresh_from_db()
+            self.assertEqual(temp.status, TempUpload.STATUS_COMPLETED)
+            self.assertEqual(temp.library_item_id is not None, True)
+            self.assertEqual(temp.e621_post_id, post_id)
+            self.assertEqual(temp.resolution, TempUpload.RESOLUTION_AUTO_MD5)
+            self.assertEqual(temp.library_item.e621_post_id, post_id)
+        self.assertEqual(MediaItem.objects.count(), 2)
+
 
 class IqdbRecordingTests(TestCase):
     """The modal needs to tell "checked, no match" from "never checked"."""