fix(share): stream single-file shares through /api/s/{token}/file/{id}

The public landing page's inline media preview (added in 6ee26e46)
requests /api/s/{token}/file/{item_id}, but assert_file_in_share went
through resolve_folder_share, which hard-rejects non-folder shares —
so for a single-file share the video src got a 400 and the player
rendered empty: the preview box appeared but nothing would play.

The AuthZ gate now branches on item_type instead: a file share only
accepts file_id == share.item_id, a folder share still requires the
file to live in the shared subtree, and anything else is NotFound
(same shape as "file doesn't exist", preserving anti-enumeration).
Password/expiry checks still happen inside
get_shared_link_with_unlock, unchanged.

Also extend public_shares.hurl section 8b: the file-share token must
stream its own item (200 + inline disposition) and reject an outsider
file id with 404.

NOTE: fmt/clippy/api-test could not run on the authoring machine (no
Rust toolchain or Docker) — run `just check` + `just api-test` before
pushing.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/application/services/share_browse_service.rs

@@ -116,19 +116,35 @@ impl ShareBrowseService {
         self.list_inner(folder_id, resolved.owner_id).await
     }
 
+    /// AuthZ gate for `/api/s/{token}/file/{file_id}`: the requested file
+    /// must either BE the shared item (single-file share — the public landing
+    /// page's inline media preview streams through here) or live inside the
+    /// shared folder's subtree (folder share). Anything else is NotFound —
+    /// the same shape as "file doesn't exist", so the endpoint can't be used
+    /// to enumerate file ids.
     pub async fn assert_file_in_share(
         &self,
         token: &str,
         file_id: &str,
         unlock_jwt: Option<&str>,
     ) -> Result<(), DomainError> {
-        let resolved = self.resolve_folder_share(token, unlock_jwt).await?;
+        let share = self
+            .share_service
+            .get_shared_link_with_unlock(token, unlock_jwt)
+            .await?;
 
-        if !self
-            .folder_repo
-            .is_file_in_subtree(file_id, &resolved.root_folder_id)
+        let in_scope = match share.item_type.as_str() {
+            // Single-file share: only the shared item itself may be streamed.
+            "file" => file_id == share.item_id,
+            // Folder share: the file must live in the shared subtree.
+            "folder" => {
+                self.folder_repo
+                    .is_file_in_subtree(file_id, &share.item_id)
                     .await?
-        {
+            }
+            _ => false,
+        };
+        if !in_scope {
             return Err(DomainError::not_found("File", file_id));
         }
         Ok(())

src/interfaces/api/handlers/share_handler.rs

@@ -639,7 +639,7 @@ pub async fn list_share_contents_subfolder(
     path = "/api/s/{token}/file/{file_id}",
     params(
         ("token" = String, Path, description = "Share token"),
-        ("file_id" = String, Path, description = "File ID (must be inside the share)")
+        ("file_id" = String, Path, description = "File ID (the shared item itself, or a file inside the shared folder's subtree)")
     ),
     responses(
         (status = 200, description = "File content (or 206 for Range request)"),

tests/api/public_shares.hurl

@@ -197,6 +197,24 @@ HTTP 200
 jsonpath "$.item_type" == "file"
 
 
+# The public landing page's inline media preview streams the shared file
+# through /api/s/{token}/file/{file_id} — the requested file IS the shared
+# item here, so the AuthZ gate must accept it (Range-aware 200, inline
+# disposition so <video>/<img> can render it).
+GET {{base_url}}/api/s/{{file_share_token}}/file/{{shared_file_id}}
+
+HTTP 200
+[Asserts]
+header "Content-Disposition" contains "hello.txt"
+
+
+# A file id that is NOT the shared item must 404 on a file-share token —
+# same anti-enumeration shape as the folder-share probe above.
+GET {{base_url}}/api/s/{{file_share_token}}/file/{{outsider_file_id}}
+
+HTTP 404
+
+
 # ─────────────────────────────────────────────────────────────
 # 9 — Mint a password-protected share on the same folder.
 # ─────────────────────────────────────────────────────────────