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/services.py

@@ -231,6 +231,7 @@ def trim_e621_post(post):
             "size": file_data.get("size"),
             "width": file_data.get("width"),
             "height": file_data.get("height"),
+            "url": file_data.get("url"),
         },
         "uploader_name": post.get("uploader_name"),
     }
@@ -244,12 +245,38 @@ def sanitize_iqdb_results(results):
             continue
         score = result.get("score")
         post_id = result.get("post_id")
+        tags_preview = result.get("tags_preview")
         entry = {
             "post_id": post_id if isinstance(post_id, int) else None,
             "score": float(score) if isinstance(score, (int, float)) else None,
             "preview_url": str(result.get("preview_url") or "")[:500] or None,
             "rating": str(result.get("rating") or "")[:1] or None,
             "md5": str(result.get("md5") or "")[:32] or None,
+            "score_total": (
+                result.get("score_total")
+                if isinstance(result.get("score_total"), int)
+                else None
+            ),
+            "fav_count": (
+                result.get("fav_count")
+                if isinstance(result.get("fav_count"), int)
+                else None
+            ),
+            "width": (
+                result.get("width")
+                if isinstance(result.get("width"), int)
+                else None
+            ),
+            "height": (
+                result.get("height")
+                if isinstance(result.get("height"), int)
+                else None
+            ),
+            "tags_preview": (
+                [str(tag)[:100] for tag in tags_preview][:8]
+                if isinstance(tags_preview, list)
+                else []
+            ),
         }
         if entry["post_id"] or entry["preview_url"]:
             cleaned.append(entry)