hscontrol/db: add migration to clear user_id on tagged nodes

Tagged nodes are owned by their tags, not a user. Previously
user_id was kept as "created by" tracking, but this prevents
deleting users whose nodes have all been tagged, and the
ON DELETE CASCADE FK would destroy the tagged nodes.

Add a migration that sets user_id = NULL on all existing tagged
nodes. Subsequent commits enforce this invariant at write time.

Updates #3077

hscontrol/db/schema.sql

@@ -79,6 +79,8 @@ CREATE TABLE nodes(
   ipv6 text,
   hostname text,
   given_name varchar(63),
+  -- user_id is NULL for tagged nodes (owned by tags, not a user).
+  -- Only set for user-owned nodes (no tags).
   user_id integer,
   register_method text,
   tags text,