feat: Problems correlation fingerprint, count, last_seen (d1-2)

- Migration 003; windowed host+type+rule correlation on ingest

- GET /problems filters; GET /problems/{id} with event timeline; ack/resolve 409 guard

- Tests and SAC_PROBLEM_CORRELATION_WINDOW_MINUTES config

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

frontend/src/api.ts

@@ -97,6 +97,9 @@ export interface ProblemSummary {
   severity: string;
   status: string;
   rule_id: string | null;
+  fingerprint?: string;
+  event_count?: number;
+  last_seen_at?: string;
   created_at: string;
   updated_at: string;
 }