Record and show that an image is already on its newest tag

An image that had been checked and is current looked exactly like one
that was never checked.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/crates/domain/src/image.rs

@@ -59,6 +59,7 @@ pub struct ImageUpdate {
     /// The image reference as it runs on the cluster.
     pub image: String,
     /// The newer tag that was checked, e.g. `docker.gitea.com/gitea:1.25.1-rootless`.
+    /// Empty when the check found no newer tag.
     pub candidate: String,
     pub checked_at: chrono::DateTime<chrono::Utc>,
     /// CVE ids that are open on the running image and gone in the candidate.
@@ -68,6 +69,13 @@ pub struct ImageUpdate {
     pub candidate_total: usize,
 }
 
+impl ImageUpdate {
+    /// True when the check found no newer tag for this image.
+    pub fn up_to_date(&self) -> bool {
+        self.candidate.is_empty()
+    }
+}
+
 /// Numeric parts of a tag plus its suffix, e.g. `1.24.2-rootless` → ([1, 24, 2], "rootless").
 fn version_parts(tag: &str) -> Option<(Vec<u64>, String)> {
     let core = tag.strip_prefix('v').unwrap_or(tag);