Answer a file id that does not exist with 404, not 500 (#162)

POST /api/enclosures/{id}/download and DELETE /api/enclosures/{id} raised a missing row with
anyhow!, which ApiError makes a 500 and logs as an api error. A page left open after a feed was
forgotten asks for exactly that: the caller's mistake, a 404 like /media/{id} already answers, and
as a 500 it turned up as a server error in the production check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Downloading or deleting a file the server no longer has answers "not found" instead of a server error.
 - A feed's page in the Directory, and subscribing from it, no longer wait while the whole Directory is put together to check that the one feed is in it.
 - The feed list, and each live update of it, no longer asks the database for a group's title once for every feed in the group.
 - An episode that came as a torrent of several files plays its largest file, and can be deleted, by hand or to free space, instead of staying on disk for good.

src/web.rs

@@ -1718,7 +1718,7 @@ async fn download_now(
         .ctx
         .db
         .enclosure(id).await?
-        .ok_or_else(|| anyhow::anyhow!("no enclosure {id}"))?;
+        .ok_or_else(|| ApiError::not_found(format!("no file {id}")))?;
     subscribed(&state, &user, &enc.feed_id).await?;
     if enc.path.is_some() {
         return Ok(StatusCode::NO_CONTENT); // Already here.
@@ -1748,7 +1748,7 @@ async fn delete_file(
         .ctx
         .db
         .enclosure(id).await?
-        .ok_or_else(|| anyhow::anyhow!("no enclosure {id}"))?;
+        .ok_or_else(|| ApiError::not_found(format!("no file {id}")))?;
     subscribed(&state, &user, &enc.feed_id).await?;
 
     // There is one copy of the file: deleting it deletes everyone's. Say so before doing