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/templates/case_page.html

@@ -164,6 +164,58 @@
                 background: #f6f6f6;
                 border-radius: 4px;
             }
+            .failure-banner {
+                margin: 1em 0;
+                padding: 1em 1.2em;
+                background: #fdecea;
+                border: 1px solid #f5c6cb;
+                border-radius: 4px;
+                color: #842029;
+            }
+            .failure-banner strong {
+                font-size: 1.05em;
+            }
+            .failure-banner .failure-time {
+                display: block;
+                margin-top: 0.4em;
+                font-size: 0.9em;
+                color: #6b1f25;
+            }
+            .failure-banner details {
+                margin-top: 0.6em;
+                font-size: 0.9em;
+            }
+            .failure-banner details summary {
+                cursor: pointer;
+                color: #6b1f25;
+            }
+            .failure-banner pre.failure-reason {
+                white-space: pre-wrap;
+                word-break: break-word;
+                margin: 0.4em 0 0 0;
+                padding: 0.6em 0.8em;
+                background: #fff;
+                border: 1px solid #f5c6cb;
+                border-radius: 3px;
+                font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
+                font-size: 0.85em;
+                color: #4a1014;
+            }
+            .failure-banner form {
+                margin: 0.8em 0 0 0;
+            }
+            .failure-banner button {
+                padding: 0.4em 1em;
+                background: #b02a37;
+                color: #fff;
+                border: none;
+                border-radius: 3px;
+                cursor: pointer;
+                font-size: 0.95em;
+            }
+            .failure-banner button:hover {
+                background: #842029;
+            }
             .recordings-link {
                 display: inline-block;
                 margin: 1em 0;
@@ -533,6 +585,33 @@
                 });
             })();
         </script>
+        {% when None %} {% match analysis_failed %} {% when Some with (failure) %}
+        <div class="failure-banner" role="alert">
+            <strong>Analyse fehlgeschlagen.</strong>
+            Bitte erneut versuchen.
+            <span class="failure-time"
+                >Zuletzt versucht:
+                <time datetime="{{ failure.failed_at }}"
+                    >{{ failure.failed_at }}</time
+                ></span
+            >
+            <details>
+                <summary>Technische Details</summary>
+                <pre class="failure-reason">{{ failure.reason }}</pre>
+            </details>
+            <form
+                method="post"
+                action="/web/cases/{{ case_id }}/analyze"
+            >
+                {% call csrf::field(csrf_token) %}
+                <input
+                    type="hidden"
+                    name="return_to"
+                    value="/web/cases/{{ case_id }}"
+                />
+                <button type="submit">Erneut analysieren</button>
+            </form>
+        </div>
         {% when None %} {% if analyzing %}
         <div class="placeholder">Wird analysiert …</div>
         {% else if recordings_count == 0 %}
@@ -542,7 +621,7 @@
             {{ recordings_count }} Aufnahme{% if recordings_count != 1 %}n{%
             endif %}, davon {{ transcribed_count }} transkribiert.
         </div>
-        {% endif %} {% endmatch %}
+        {% endif %} {% endmatch %} {% endmatch %}
 
         <div>
             <a
@@ -567,6 +646,20 @@
                     TimeFmt.dateLabel(d) + " - " + TimeFmt.timeLabel(d);
             })();
 
+            // Same locale treatment for the failure-banner timestamp.
+            // No-JS fallback is the raw RFC3339 UTC string the server
+            // rendered into the <time> element.
+            (() => {
+                const el = document.querySelector(
+                    ".failure-banner time[datetime]",
+                );
+                if (!el) return;
+                const d = new Date(el.dateTime);
+                if (isNaN(d.getTime())) return;
+                el.textContent =
+                    TimeFmt.dateLabel(d) + " " + TimeFmt.timeLabel(d);
+            })();
+
             (() => {
                 const cb = document.getElementById("admin-view-toggle");
                 if (!cb) return;