diff --git a/backend/app/api/extension.py b/backend/app/api/extension.py
index 20b1b30..082bc1e 100644
--- a/backend/app/api/extension.py
+++ b/backend/app/api/extension.py
@@ -6,6 +6,7 @@ from __future__ import annotations
 
 import asyncio
 import hashlib
+import hmac
 import re
 from pathlib import Path
 
@@ -41,7 +42,15 @@ async def _ext_key_required(session) -> bool:
     stored = (await session.execute(
         select(AppSetting.value).where(AppSetting.key == "extension_api_key")
     )).scalar_one_or_none()
-    return stored is not None and supplied == stored
+    if stored is None:
+        return False
+    # compare_digest, not `==`: the stored key is a shared secret, and a
+    # short-circuiting compare leaks its prefix through timing. Costs nothing
+    # here — it is not that this route is exposed (#3072). Compared as BYTES:
+    # compare_digest's str form rejects non-ASCII with TypeError, and this
+    # header is attacker-supplied, so a str compare would turn a junk key into
+    # a 500 instead of a 403.
+    return hmac.compare_digest(supplied.encode("utf-8"), stored.encode("utf-8"))
 
 
 def _extract_version(xpi_name: str) -> str:
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index f06d74d..a11d267 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -27,7 +27,7 @@ from .patreon_seen_media import PatreonSeenMedia
 from .pixiv_failed_media import PixivFailedMedia
 from .pixiv_seen_media import PixivSeenMedia
 from .post import Post
-from .post_attachment import PostAttachment
+from .post_attachment import PostAttachment, attachment_download_url
 from .presentation_review import PresentationReview
 from .series_chapter import SeriesChapter
 from .series_page import SeriesPage
@@ -58,6 +58,7 @@ __all__ = [
     "SubscribeStarSeenMedia",
     "Post",
     "PostAttachment",
+    "attachment_download_url",
     "PresentationReview",
     "SeriesChapter",
     "SeriesPage",
diff --git a/backend/app/models/post_attachment.py b/backend/app/models/post_attachment.py
index 1edb7f3..f522cdf 100644
--- a/backend/app/models/post_attachment.py
+++ b/backend/app/models/post_attachment.py
@@ -65,3 +65,15 @@ class PostAttachment(Base):
     captured_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )
+
+
+def attachment_download_url(attachment_id: int) -> str:
+    """The path that streams this attachment's bytes.
+
+    Both serializers that expose an attachment to the frontend
+    (`provenance_service`, `post_feed_service`) built this literal themselves,
+    so changing the route in `api/attachments.py` meant two edits and only one
+    would be remembered (#3072). `test_attachment_download_url` pins it against
+    the app's registered rule, so the drift is caught rather than trusted to.
+    """
+    return f"/api/attachments/{attachment_id}/download"
diff --git a/backend/app/services/post_feed_service.py b/backend/app/services/post_feed_service.py
index c1304ac..d4aec83 100644
--- a/backend/app/services/post_feed_service.py
+++ b/backend/app/services/post_feed_service.py
@@ -24,6 +24,7 @@ from ..models import (
     Post,
     PostAttachment,
     Source,
+    attachment_download_url,
 )
 from ..utils.html_sanitize import (
     extract_img_srcs,
@@ -360,7 +361,7 @@ class PostFeedService:
                 "ext": att.ext,
                 "mime": att.mime,
                 "size_bytes": att.size_bytes,
-                "download_url": f"/api/attachments/{att.id}/download",
+                "download_url": attachment_download_url(att.id),
             })
         return out
 
diff --git a/backend/app/services/provenance_service.py b/backend/app/services/provenance_service.py
index 6fee555..02f1a64 100644
--- a/backend/app/services/provenance_service.py
+++ b/backend/app/services/provenance_service.py
@@ -16,6 +16,7 @@ from ..models import (
     Post,
     PostAttachment,
     Source,
+    attachment_download_url,
 )
 from ..utils.html_sanitize import sanitize_post_html
 
@@ -53,7 +54,7 @@ def _attachment_dict(a: PostAttachment) -> dict:
         "original_filename": a.original_filename,
         "size_bytes": a.size_bytes,
         "ext": a.ext,
-        "download_url": f"/api/attachments/{a.id}/download",
+        "download_url": attachment_download_url(a.id),
     }
 
 
diff --git a/frontend/src/components/gallery/GalleryItem.vue b/frontend/src/components/gallery/GalleryItem.vue
index cab8b9e..f5fd815 100644
--- a/frontend/src/components/gallery/GalleryItem.vue
+++ b/frontend/src/components/gallery/GalleryItem.vue
@@ -139,9 +139,9 @@ function onThumbError() { thumbError.value = true }
   position: absolute; top: 8px; left: 8px;
   width: 22px; height: 22px; border-radius: 4px;
   border: 2px solid rgba(232, 228, 216, 0.8);
-  background: rgba(20, 23, 26, 0.45);
+  background: rgba(var(--v-theme-background), 0.45);
   display: grid; place-items: center;
-  color: #14171A; z-index: 11;
+  color: rgb(var(--v-theme-background)); z-index: 11;
 }
 .fc-gallery-item__checkbox.on {
   background: rgb(var(--v-theme-accent));
@@ -152,7 +152,7 @@ function onThumbError() { thumbError.value = true }
   min-width: 22px; height: 22px; padding: 0 5px;
   border-radius: 11px;
   background: rgb(var(--v-theme-accent));
-  color: #14171A; font-size: 12px; font-weight: 700;
+  color: rgb(var(--v-theme-background)); font-size: 12px; font-weight: 700;
   display: grid; place-items: center; z-index: 11;
   pointer-events: none;
 }
@@ -160,7 +160,8 @@ function onThumbError() { thumbError.value = true }
   position: absolute; left: 0; right: 0; bottom: 0;
   padding: 14px 8px 6px;
   background: linear-gradient(
-    to top, rgba(20, 23, 26, 0.78), rgba(20, 23, 26, 0)
+    to top, rgba(var(--v-theme-background), 0.78),
+    rgba(var(--v-theme-background), 0)
   );
   font-size: 12px; line-height: 1.2;
   white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
diff --git a/frontend/src/components/settings/DownloadsActivityPanel.vue b/frontend/src/components/settings/DownloadsActivityPanel.vue
index d8f9d16..380b44a 100644
--- a/frontend/src/components/settings/DownloadsActivityPanel.vue
+++ b/frontend/src/components/settings/DownloadsActivityPanel.vue
@@ -35,7 +35,7 @@
         All subscription sources healthy.
       </p>
       <p v-else class="text-body-2 mb-0">
-        <b class="fc-bad">{{ failing.length }}</b> failing source(s):
+        <b class="fc-weak">{{ failing.length }}</b> failing source(s):
         <span class="fc-muted">{{ failingNames }}</span>
       </p>
     </v-card-text>
@@ -72,5 +72,4 @@ onUnmounted(() => { if (pollId) clearInterval(pollId) })
 </script>
 
 <style scoped>
-.fc-bad { color: rgb(var(--v-theme-error)); }
 </style>
diff --git a/frontend/src/components/settings/GpuActivityPanel.vue b/frontend/src/components/settings/GpuActivityPanel.vue
index 39bba29..1f85a5b 100644
--- a/frontend/src/components/settings/GpuActivityPanel.vue
+++ b/frontend/src/components/settings/GpuActivityPanel.vue
@@ -25,7 +25,7 @@
           <div class="fc-cell__l">done</div>
         </div>
         <div class="fc-cell">
-          <div class="fc-cell__n" :class="q.error ? 'fc-bad' : ''">{{ q.error }}</div>
+          <div class="fc-cell__n" :class="q.error ? 'fc-weak' : ''">{{ q.error }}</div>
           <div class="fc-cell__l">errored</div>
         </div>
       </div>
@@ -104,5 +104,4 @@ onUnmounted(() => { if (pollId) clearInterval(pollId) })
   font-size: 11px; text-transform: uppercase; letter-spacing: 0.04em;
   color: rgb(var(--v-theme-on-surface-variant));
 }
-.fc-bad { color: rgb(var(--v-theme-error)); }
 </style>
diff --git a/frontend/src/styles/app.css b/frontend/src/styles/app.css
index 3e24a75..fbd9380 100644
--- a/frontend/src/styles/app.css
+++ b/frontend/src/styles/app.css
@@ -50,7 +50,12 @@
 
 /* Status text colours (DRY pass #161): fc-good = success, fc-weak = error,
    consolidated from the GPU / heads cards. fc-ok is intentionally NOT global —
-   it means on-surface in HeadsCard but success in QueuesTable. */
+   it means on-surface in HeadsCard but success in QueuesTable.
+
+   No `.fc-bad` (#3072): it was defined locally and identically in the Downloads
+   and GPU activity panels, and it is fc-weak under a second name — GpuAgentCard
+   and GpuActivityPanel were colouring the same "errored" count with different
+   class names. Both now use fc-weak. Reach for fc-weak, not a new synonym. */
 .fc-good { color: rgb(var(--v-theme-success)); }
 .fc-weak { color: rgb(var(--v-theme-error)); }
 
diff --git a/tests/test_api_attachments.py b/tests/test_api_attachments.py
index 95e31f2..7918008 100644
--- a/tests/test_api_attachments.py
+++ b/tests/test_api_attachments.py
@@ -1,6 +1,6 @@
 import pytest
 
-from backend.app.models import Artist, PostAttachment
+from backend.app.models import Artist, PostAttachment, attachment_download_url
 
 pytestmark = pytest.mark.integration
 
@@ -33,3 +33,16 @@ async def test_download_streams_with_disposition(client, db, tmp_path):
 async def test_download_404(client):
     resp = await client.get("/api/attachments/999999/download")
     assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_attachment_download_url_routes_to_the_download_endpoint(app):
+    """The two serializers no longer hand-format this path (#3072) — but a
+    single definition is only worth having if it still matches the route. Pin
+    it by MATCHING against the real URL map rather than comparing to a literal:
+    a string equality test would pass just as happily after someone renamed the
+    route, which is the exact drift the helper exists to prevent."""
+    built = attachment_download_url(4242)
+    endpoint, args = app.url_map.bind("localhost").match(built)
+    assert endpoint == "attachments.download"
+    assert args == {"attachment_id": 4242}