feat: add cached DNS checks

backend/alembic/env.py

@@ -21,6 +21,7 @@ if database_url:
     config.set_main_option("sqlalchemy.url", _make_sync_db_url(database_url))
 
 import app.models.alert  # noqa: E402, F401
+import app.models.dns_cache  # noqa: E402, F401
 import app.models.domain  # noqa: E402, F401
 import app.models.mail_source  # noqa: E402, F401
 import app.models.mail_source_import  # noqa: E402, F401

backend/alembic/versions/b8c9d0e1f2a3_add_dns_cache.py

@@ -0,0 +1,60 @@
+"""add dns cache
+
+Revision ID: b8c9d0e1f2a3
+Revises: a7b8c9d0e1f2
+Create Date: 2026-05-22 23:28:00.000000
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "b8c9d0e1f2a3"
+down_revision: Union[str, Sequence[str], None] = "a7b8c9d0e1f2"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Create a DNS result cache table."""
+    op.create_table(
+        "dns_cache",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("domain", sa.String(), nullable=False),
+        sa.Column("provider", sa.String(), nullable=False),
+        sa.Column("selectors_key", sa.String(length=64), nullable=False),
+        sa.Column("result_json", sa.Text(), nullable=False),
+        sa.Column("checked_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("domain", "provider", "selectors_key", name="uq_dns_cache_lookup"),
+    )
+    op.create_index(op.f("ix_dns_cache_id"), "dns_cache", ["id"], unique=False)
+    op.create_index(op.f("ix_dns_cache_domain"), "dns_cache", ["domain"], unique=False)
+    op.create_index(op.f("ix_dns_cache_provider"), "dns_cache", ["provider"], unique=False)
+    op.create_index(
+        op.f("ix_dns_cache_selectors_key"),
+        "dns_cache",
+        ["selectors_key"],
+        unique=False,
+    )
+    op.create_index(op.f("ix_dns_cache_checked_at"), "dns_cache", ["checked_at"], unique=False)
+    op.create_index(
+        "ix_dns_cache_domain_checked",
+        "dns_cache",
+        ["domain", "checked_at"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    """Drop the DNS result cache table."""
+    op.drop_index("ix_dns_cache_domain_checked", table_name="dns_cache")
+    op.drop_index(op.f("ix_dns_cache_checked_at"), table_name="dns_cache")
+    op.drop_index(op.f("ix_dns_cache_selectors_key"), table_name="dns_cache")
+    op.drop_index(op.f("ix_dns_cache_provider"), table_name="dns_cache")
+    op.drop_index(op.f("ix_dns_cache_domain"), table_name="dns_cache")
+    op.drop_index(op.f("ix_dns_cache_id"), table_name="dns_cache")
+    op.drop_table("dns_cache")