feat(media): authenticated signed media gateway (C1, C2-sink, C3, H2, H10)

Replaces the DB-blind `/media` ServeDir with a signed, DB-aware gateway at
`GET /media/{kind}/{id}`. Every media byte now flows through an HMAC-SHA256
signature check (minted into feed/upload DTOs for authenticated members; <img>
can't carry a Bearer header) plus a DB lookup:

- C1: export ZIP/HTML have no upload row, so they are unreachable by path —
  download stays behind the authenticated /export endpoints.
- C2 (sink): responses carry X-Content-Type-Options: nosniff and a locked-down
  CSP (default-src 'none'; sandbox), neutralizing any active content.
- C3 / H2: find_by_id filters deleted_at and the handler rejects ban-hidden
  uploaders, so deleted and moderated artifacts 404 — and the unauthenticated
  get_original alias (the H2 hole) is removed entirely.
- H10: delete paths (owner + host) now unlink original/preview/thumbnail after
  commit; soft_delete returns the paths; an hourly reaper reclaims disk for
  rows soft-deleted past a 1-day grace and hard-deletes them (FKs cascade).

Signed URLs are bucketed to a 1h window so they stay stable across feed polls
(browser cache hits) while expiring within 24h. media_token sign/verify has a
unit test (roundtrip + tamper + expiry).

Frontend: FeedUpload/pickMediaUrl now use the backend-provided signed
original_url; no client constructs a media path anymore.

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

backend/src/handlers/feed.rs

@@ -10,6 +10,7 @@ use uuid::Uuid;
 use crate::auth::middleware::AuthUser;
 use crate::error::AppError;
 use crate::services::config;
+use crate::services::media_token;
 use crate::services::rate_limiter::client_ip;
 use crate::state::AppState;
 
@@ -27,6 +28,8 @@ pub struct FeedUpload {
     pub uploader_name: String,
     pub preview_url: Option<String>,
     pub thumbnail_url: Option<String>,
+    /// Signed gateway URL for the full-resolution original. Always present.
+    pub original_url: Option<String>,
     pub mime_type: String,
     pub caption: Option<String>,
     pub like_count: i64,
@@ -55,6 +58,33 @@ struct FeedRow {
     created_at: DateTime<Utc>,
 }
 
+/// Build a feed DTO, minting fresh signed gateway URLs for each artifact. The
+/// preview/thumbnail URLs are present only when the derivative exists so the
+/// client can show a skeleton while compression is still running; the original
+/// URL is always present.
+fn to_feed_upload(r: FeedRow, liked: bool, secret: &str, now: i64) -> FeedUpload {
+    FeedUpload {
+        liked_by_me: liked,
+        preview_url: r
+            .preview_path
+            .as_ref()
+            .map(|_| media_token::signed_url(secret, "preview", r.id, now)),
+        thumbnail_url: r
+            .thumbnail_path
+            .as_ref()
+            .map(|_| media_token::signed_url(secret, "thumbnail", r.id, now)),
+        original_url: Some(media_token::signed_url(secret, "original", r.id, now)),
+        id: r.id,
+        user_id: r.user_id,
+        uploader_name: r.uploader_name,
+        mime_type: r.mime_type,
+        caption: r.caption,
+        like_count: r.like_count,
+        comment_count: r.comment_count,
+        created_at: r.created_at,
+    }
+}
+
 pub async fn feed(
     State(state): State<AppState>,
     auth: AuthUser,
@@ -135,24 +165,13 @@ pub async fn feed(
     let upload_ids: Vec<Uuid> = rows.iter().map(|r| r.id).collect();
     let liked_set = get_liked_set(&state.pool, auth.user_id, &upload_ids).await;
 
+    let now = Utc::now().timestamp();
+    let secret = &state.config.jwt_secret;
     let uploads = rows
         .into_iter()
         .map(|r| {
-            let preview_url = r.preview_path.map(|p| format!("/media/{p}"));
-            let thumbnail_url = r.thumbnail_path.map(|p| format!("/media/{p}"));
-            FeedUpload {
-                liked_by_me: liked_set.contains(&r.id),
-                id: r.id,
-                user_id: r.user_id,
-                uploader_name: r.uploader_name,
-                preview_url,
-                thumbnail_url,
-                mime_type: r.mime_type,
-                caption: r.caption,
-                like_count: r.like_count,
-                comment_count: r.comment_count,
-                created_at: r.created_at,
-            }
+            let liked = liked_set.contains(&r.id);
+            to_feed_upload(r, liked, secret, now)
         })
         .collect();
 
@@ -202,20 +221,13 @@ pub async fn feed_delta(
     let upload_ids: Vec<Uuid> = rows.iter().map(|r| r.id).collect();
     let liked_set = get_liked_set(&state.pool, auth.user_id, &upload_ids).await;
 
+    let now = Utc::now().timestamp();
+    let secret = &state.config.jwt_secret;
     let uploads = rows
         .into_iter()
-        .map(|r| FeedUpload {
-            liked_by_me: liked_set.contains(&r.id),
-            id: r.id,
-            user_id: r.user_id,
-            uploader_name: r.uploader_name,
-            preview_url: r.preview_path.map(|p| format!("/media/{p}")),
-            thumbnail_url: r.thumbnail_path.map(|p| format!("/media/{p}")),
-            mime_type: r.mime_type,
-            caption: r.caption,
-            like_count: r.like_count,
-            comment_count: r.comment_count,
-            created_at: r.created_at,
+        .map(|r| {
+            let liked = liked_set.contains(&r.id);
+            to_feed_upload(r, liked, secret, now)
         })
         .collect();