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/models/problem.py

@@ -1,6 +1,6 @@
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, String, Text, func
+from sqlalchemy import DateTime, ForeignKey, Integer, String, Text, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
@@ -16,6 +16,9 @@ class Problem(Base):
     severity: Mapped[str] = mapped_column(String(16), index=True)
     status: Mapped[str] = mapped_column(String(32), index=True, default="open")
     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)
+    last_seen_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()