Split the vulnerability view into host and container categories

The findings of the Debian host (OS packages and applications found in the
root filesystem) and of the container images in the cluster were mixed in
one flat table. They are now two prominent categories: a card each with its
own severity split and target count, acting as the primary selector, and a
table that adapts to the selection. Host findings show the package source
reported by the scanner (debian, gobinary, node-pkg …) instead of the
target, container findings show the image.

Backend: findings carry the scanner's package source, the list endpoint
takes ?scope=host|container, and the targets endpoint reports each target
with its category and open count.

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

backend/crates/application/src/test_fakes.rs

@@ -525,6 +525,7 @@ pub fn raw(
         fixed_version: fixed.map(String::from),
         title: format!("{cve} in {pkg}"),
         url: format!("https://nvd.nist.gov/vuln/detail/{cve}"),
+        source: "debian".into(),
     }
 }
 
@@ -627,6 +628,7 @@ impl FindingRepository for MemFindings {
             .iter()
             .filter(|f| filter.include_fixed || f.status != FindingStatus::Fixed)
             .filter(|f| filter.min_severity.is_none_or(|m| f.raw.severity >= m))
+            .filter(|f| filter.target_kind.is_none_or(|k| f.target_kind == k))
             .filter(|f| filter.target.as_ref().is_none_or(|t| &f.target == t))
             .filter(|f| filter.status.is_none_or(|s| f.status == s))
             .cloned()