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>

backend/app/config.py

@@ -53,6 +53,9 @@ class Settings(BaseSettings):
     # Порог «живости» агента: agent.heartbeat раз в ~12 ч (ssh-monitor)
     sac_heartbeat_stale_minutes: int = 780
 
+    # Окно корреляции Problems: host + type + rule в одном open Problem
+    sac_problem_correlation_window_minutes: int = 60
+
 
 @lru_cache
 def get_settings() -> Settings: