fix: explicit milliseconds precision of timestamp columns

scripts/clean_duplicates.sql

@@ -0,0 +1,12 @@
+DELETE t1
+FROM heartbeats t1
+         INNER JOIN heartbeats t2
+WHERE t1.id < t2.id
+  AND t1.time = t2.time
+  AND t1.entity = t2.entity
+  AND t1.is_write = t2.is_write
+  AND t1.branch = t2.branch
+  AND t1.editor = t2.editor
+  AND t1.machine = t2.machine
+  AND t1.operating_system = t2.operating_system
+  AND t1.user_id = t2.user_id;