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/serializers.py

@@ -3,7 +3,6 @@ from datetime import timedelta
 from pathlib import Path
 
 from django.conf import settings
-from django.core import signing
 from rest_framework import serializers
 
 from .models import (
@@ -20,6 +19,7 @@ from .services import (
     VIDEO_EXTENSIONS,
     signed_media_url,
 )
+from .signing_urls import sign_payload
 
 
 class MediaLocationSerializer(serializers.ModelSerializer):
@@ -199,9 +199,9 @@ class TempUploadSerializer(serializers.ModelSerializer):
         user = self._request_user()
         if user is None:
             return None
-        signature = signing.dumps(
+        signature = sign_payload(
             {"temp": str(obj.id), "user": user.id},
-            salt=UPLOAD_FILE_SALT,
+            UPLOAD_FILE_SALT,
         )
         url = f"/api/uploads/{obj.id}/file/?sig={signature}"
         request = self.context.get("request")
@@ -339,9 +339,9 @@ class SimilarityCheckSerializer(serializers.ModelSerializer):
         user = self._request_user()
         if user is None or not obj.file:
             return None
-        signature = signing.dumps(
+        signature = sign_payload(
             {"check": str(obj.id), "user": user.id},
-            salt=UPLOAD_FILE_SALT,
+            UPLOAD_FILE_SALT,
         )
         url = f"/api/similarity/{obj.id}/file/?sig={signature}"
         request = self.context.get("request")