fix: 12h cooldown before reopening manually closed host_silence (0.9.9)

Track resolved_by on problems; suppress scan/ingest recreate after manual
resolve; reopen same problem after cooldown; auto-close via heartbeat unchanged.

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

backend/app/models/problem.py

@@ -15,6 +15,7 @@ class Problem(Base):
     summary: Mapped[str] = mapped_column(Text)
     severity: Mapped[str] = mapped_column(String(16), index=True)
     status: Mapped[str] = mapped_column(String(32), index=True, default="open")
+    resolved_by: Mapped[str | None] = mapped_column(String(16))
     rule_id: Mapped[str | None] = mapped_column(String(64))
     fingerprint: Mapped[str] = mapped_column(String(128), index=True)
     event_count: Mapped[int] = mapped_column(Integer, default=1)