Enrich IQDB candidates; link downloads the e621 original

- IQDB responses carry no preview/file data, so candidates only showed an
  ID; the SPA now enriches them with one batched posts lookup (preview,
  rating, score, favourites, dimensions, tag preview)
- Candidate tiles are selectable instead of instantly resolving: picking
  one shows its info and an explicit 'Link selected post' button
- Linking a post now fetches the e621 original into the library and
  drops the staged upload; when the staged file's MD5 already equals the
  post's file, the staged copy is moved instead (identical bytes)
- Keep the file URL in stored e621 metadata; sanitize the new candidate
  fields server-side

backend/apps/library/uploads.py

@@ -12,6 +12,7 @@ Flow:
 import logging
 import shutil
 from pathlib import Path
+from urllib.parse import urlparse
 
 from django.conf import settings
 from django.contrib.auth import get_user_model
@@ -31,20 +32,43 @@ from .serializers import TempUploadSerializer
 logger = logging.getLogger(__name__)
 
 
-def complete_temp_upload(temp):
-    """Move a staged file into the watched folder and index it."""
+def complete_temp_upload(temp, download_url=None):
+    """Index the upload into the library.
+
+    With a ``download_url`` the e621 original is fetched and the staged copy
+    is dropped; otherwise the staged file itself moves into the library.
+    """
     folder = Path(settings.WATCHED_FOLDER)
-    if not temp.file:
-        raise ValueError("staged file is missing")
 
-    destination = services.unique_destination(
-        folder, temp.original_filename or "upload"
-    )
-    with temp.file.open("rb") as source, open(destination, "wb") as target:
-        shutil.copyfileobj(source, target)
+    if download_url:
+        file_data = {}
+        if isinstance(temp.e621_data, dict):
+            file_data = temp.e621_data.get("file") or {}
+        extension = Path(urlparse(download_url).path).suffix
+        if not extension:
+            extension = Path(temp.original_filename).suffix
+        md5 = file_data.get("md5")
+        name = f"{md5}{extension}" if md5 else (temp.original_filename or "download")
+        destination = services.unique_destination(folder, name)
+        try:
+            services.download_file(download_url, destination)
+        except Exception:
+            destination.unlink(missing_ok=True)
+            raise
+        item, _, _ = services.index_file(destination, folder)
+        if temp.file:
+            temp.file.delete(save=False)
+    else:
+        if not temp.file:
+            raise ValueError("staged file is missing")
+        destination = services.unique_destination(
+            folder, temp.original_filename or "upload"
+        )
+        with temp.file.open("rb") as source, open(destination, "wb") as target:
+            shutil.copyfileobj(source, target)
+        item, _, _ = services.index_file(destination, folder)
+        temp.file.delete(save=False)
 
-    item, _, _ = services.index_file(destination, folder)
-    temp.file.delete(save=False)
     temp.library_item = item
     temp.status = TempUpload.STATUS_COMPLETED
 
@@ -184,6 +208,7 @@ class TempUploadViewSet(
             )
 
         mode = str(request.data.get("mode") or "").strip()
+        download_url = None
         if mode == "link":
             post = request.data.get("post")
             post_id = request.data.get("post_id")
@@ -213,6 +238,17 @@ class TempUploadViewSet(
             notes = request.data.get("notes")
             if notes is not None:
                 temp.custom_notes = str(notes)
+
+            # Prefer the e621 original; fall back to the staged file when the
+            # post has no URL or its file is byte-identical to the upload.
+            file_data = (trimmed or {}).get("file") or {}
+            candidate_url = str(
+                request.data.get("file_url") or file_data.get("url") or ""
+            ).strip()
+            post_md5 = str(file_data.get("md5") or "").strip().lower()
+            if candidate_url and post_md5 and post_md5 == temp.md5.lower():
+                candidate_url = ""
+            download_url = candidate_url or None
         elif mode == "custom":
             rating = str(request.data.get("rating") or "")
             temp.custom_rating = rating if rating in {"s", "q", "e"} else ""
@@ -227,7 +263,7 @@ class TempUploadViewSet(
 
         temp.save()
         try:
-            complete_temp_upload(temp)
+            complete_temp_upload(temp, download_url=download_url)
         except Exception as exc:  # noqa: BLE001 - report completion failures
             logger.exception("Could not complete staged upload %s", temp.id)
             temp.status = TempUpload.STATUS_ERROR