Make media URLs stable and cacheable, add real image thumbnails

Signed media URLs embedded the current second (TimestampSigner), so every
API response re-minted every raw/thumbnail/staged URL and the browser
re-downloaded each file on every poll or navigation. Responses also carried
no cache headers at all.

- sign with a plain Signer plus a bucket-quantized exp (7d TTL, 24h bucket),
  so a URL is byte-identical across responses and rotates once a day; legacy
  TimestampSigner URLs stay accepted for one release
- add a v=<md5> version parameter to library media URLs so replacing a file
  under the same J-ID (the optimize flow) busts caches exactly when needed
- serve_file now sends ETag/Last-Modified and a private Cache-Control and
  answers conditional requests with 304; library media gets max-age 6d +
  immutable, staged/similarity files 1h
- build cached 480px JPEG thumbnails for images (Pillow, keyed by MD5 under
  MEDIA_ROOT/thumbs) instead of serving full-size originals through the
  thumbnail endpoint; the library grid uses thumbnail_url for images too

backend/apps/library/similarity.py

@@ -11,7 +11,6 @@ from datetime import timedelta
 from pathlib import Path
 
 from django.conf import settings
-from django.core import signing
 from django.utils import timezone
 from rest_framework import mixins, status, viewsets
 from rest_framework.decorators import action
@@ -22,6 +21,7 @@ from rest_framework.response import Response
 from . import services
 from .models import MediaItem, SimilarityCheck
 from .serializers import SimilarityCheckSerializer
+from .signing_urls import load_payload
 from .tools import item_brief
 from .uploads import find_library_matches
 
@@ -149,14 +149,7 @@ class SimilarityCheckViewSet(
             check = self.get_queryset().filter(pk=pk).first()
         else:
             signature = request.query_params.get("sig")
-            payload = None
-            if signature:
-                try:
-                    payload = signing.loads(
-                        signature, salt=services.UPLOAD_FILE_SALT, max_age=86400
-                    )
-                except signing.BadSignature:
-                    payload = None
+            payload = load_payload(signature, services.UPLOAD_FILE_SALT) if signature else None
             if payload and payload.get("check") == str(pk):
                 check = SimilarityCheck.objects.filter(pk=pk).first()
         if check is None or not check.file: