Improve warning when harvest item cannot be deserialized

This is a band aid fix and not perfect. When I added the `Unknown`
variant to the enum, I wasn't aware that it was used when other variants
fail to deserialize even with `kind: "event"`. So the warning there
was misleading.

It's still not perfect because I ideally want to print more information
on the deseralization failure. And the `kind` strings are duplicated
now. I think in the future, the `HarvestResponse` might have items of
type `{ kind: String, #[serde(flatten)] body: serde_json::Value }` or
something like that. And then we deserialize it manually in a second
step. We will see.

backend/src/sync/harvest/mod.rs

@@ -323,9 +323,23 @@ async fn store_in_db(
                 removed_playlists += 1;
             }
 
-            HarvestItem::Unknown { kind, .. } => {
-                warn!("Unknown item of kind '{kind}' in harvest response. \
-                    You might need to update Tobira.");
+            HarvestItem::Unknown { kind, updated } => {
+                let known = [
+                    "event",
+                    "event-deleted",
+                    "series",
+                    "series-deleted",
+                    "playlist",
+                    "playlist-deleted",
+                ];
+
+                if known.contains(&&*kind) {
+                    warn!("Could not deserialize item in harvest response for \
+                        kind '{kind}' (updated {updated})");
+                } else {
+                    warn!("Unknown item of kind '{kind}' in harvest response. \
+                        You might need to update Tobira.");
+                }
             }
         }
     }