feat: initial MailOne worker (latest email per recipient, 24h retention)

schema.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS latest_emails (
+  recipient TEXT PRIMARY KEY,
+  id TEXT,
+  sender TEXT,
+  nexthop TEXT,
+  subject TEXT,
+  content TEXT,
+  received_at INTEGER NOT NULL
+);