feat: Show LLM failure banner and retry button

When an LLM analysis fails, a `.analysis_failed.json` marker is created.
If no document exists yet and the auto-trigger is blocked by this
marker,
the case page must display a banner. This banner provides the failure
reason and a button to retry the analysis, ensuring users are not left
in a dead-end state.

The `read_failure_marker` function is made public to allow the web layer
to access this failure information. The `CasePageTemplate` is updated to
include `analysis_failed` data, which conditionally renders the new
`.failure-banner` HTML.

This change prevents cases from becoming unrecoverable due to transient
LLM errors.

server/src/analyze/auto_trigger.rs

@@ -269,7 +269,13 @@ async fn scan_m4as(case_dir: &Path) -> M4aScan {
     scan
 }
 
-async fn read_failure_marker(case_dir: &Path) -> Option<FailureMarker> {
+/// Read and parse the per-case failure marker. `pub(crate)` so the web
+/// layer can surface the failure state in the case page (otherwise a
+/// transport-level LLM error leaves the user with no UI affordance to
+/// retry — the auto-trigger sees the marker and skips, but no document
+/// exists yet either, so the existing `Neu analysieren` button is not
+/// rendered).
+pub(crate) async fn read_failure_marker(case_dir: &Path) -> Option<FailureMarker> {
     let bytes = tokio::fs::read(case_dir.join(FAILURE_MARKER_FILE))
         .await
         .ok()?;