feat: add cached DNS checks

backend/app/models/dns_cache.py

@@ -0,0 +1,30 @@
+from datetime import UTC, datetime
+
+from sqlalchemy import Column, DateTime, Index, Integer, String, Text, UniqueConstraint
+
+from app.core.database import Base
+
+
+def _utcnow_naive() -> datetime:
+    return datetime.now(UTC).replace(tzinfo=None)
+
+
+class DNSCache(Base):
+    """Cached DNS authentication result for a domain and selector set."""
+
+    __tablename__ = "dns_cache"
+
+    id = Column(Integer, primary_key=True, index=True)
+    domain = Column(String, nullable=False, index=True)
+    provider = Column(String, nullable=False, index=True)
+    selectors_key = Column(String(64), nullable=False, index=True)
+    result_json = Column(Text, nullable=False)
+    checked_at = Column(DateTime, default=_utcnow_naive, nullable=False, index=True)
+
+    __table_args__ = (
+        UniqueConstraint("domain", "provider", "selectors_key", name="uq_dns_cache_lookup"),
+        Index("ix_dns_cache_domain_checked", "domain", "checked_at"),
+    )
+
+    def __repr__(self):
+        return f"<DNSCache {self.domain} provider={self.provider}>"