feat: UI Problems list filters, detail card and event timeline (d2-1)

- Filters status/severity/hostname; /problems/:id with Ack/Resolve and timeline

- Dashboard drill-down to open problems

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/api.ts

@@ -111,6 +111,24 @@ export interface ProblemListResponse {
   page_size: number;
 }
 
+export interface ProblemEventItem {
+  id: number;
+  event_id: string;
+  occurred_at: string;
+  type: string;
+  severity: string;
+  title: string;
+  summary: string;
+}
+
+export interface ProblemDetail extends ProblemSummary {
+  events: ProblemEventItem[];
+}
+
+export function fetchProblem(id: number): Promise<ProblemDetail> {
+  return apiFetch<ProblemDetail>(`/api/v1/problems/${id}`);
+}
+
 export async function ackProblem(problemId: number): Promise<{ id: number; status: string }> {
   return apiFetch(`/api/v1/problems/${problemId}/ack`, { method: "POST" });
 }