fix(security): re-review follow-ups — drop HEIC/HEIF, fix stale comment

Address the two items from the adversarial re-review of batch-2.

- upload: remove image/heic + image/heif from ALLOWED_MEDIA. Neither the
  `image` crate nor the bundled ffmpeg 6.1 (Alpine 3.21 — HEIF demuxer
  only landed in ffmpeg 7.0) can decode them, so accepting them stored
  posts that never got a thumbnail. iOS Safari transcodes HEIC->JPEG on
  file-input selection, so this rejects only the rare HEIC-preserving
  path, now with a clear error instead of a silently broken post.
- data-mode-store: correct the stale "auth-gated" comment — the
  /original route is intentionally unauthenticated (UUID-as-capability)
  so it works from plain <img src> / <video src>.

Re-verified: cargo build (only the pre-existing middleware.rs warning).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/handlers/upload.rs

@@ -21,13 +21,17 @@ const MAX_CAPTION_LENGTH: usize = 2000;
 /// we trust, store, and hand to the compression pipeline — so a text-based payload
 /// (SVG/HTML/JS) can never be stored or served on-origin. Each entry maps to the
 /// server-controlled file extension we persist the original under.
+///
+/// HEIC/HEIF are deliberately excluded: the preview pipeline (`image` crate, and
+/// the bundled ffmpeg 6.1) cannot decode them, so accepting them would store files
+/// that never get a thumbnail. iOS Safari already transcodes HEIC→JPEG when a photo
+/// is selected via a file input, so this rejects only the rare HEIC-preserving
+/// upload path — with a clear error rather than a silently broken post.
 const ALLOWED_MEDIA: &[(&str, &str)] = &[
     ("image/jpeg", "jpg"),
     ("image/png", "png"),
     ("image/webp", "webp"),
     ("image/gif", "gif"),
-    ("image/heic", "heic"),
-    ("image/heif", "heif"),
     ("video/mp4", "mp4"),
     ("video/quicktime", "mov"),
     ("video/webm", "webm"),