database: added keys migration, get/insert, insert keys at launch if none are present

crates/database/queries/keys/get_most_recent.sql

@@ -0,0 +1,10 @@
+select id,
+       created_at as "created_at: DateTime<Utc>",
+       revoked_at as "revoked_at: DateTime<Utc>",
+       private_der,
+       public_der
+
+from keys
+where revoked_at is null
+order by created_at desc
+limit 1