Tagged nodes are owned by their tags, not a user. Enforce this
invariant at every write path:
- createAndSaveNewNode: do not set UserID for tagged PreAuthKey
registration; clear UserID when advertise-tags are applied
during OIDC/CLI registration
- SetNodeTags: clear UserID/User when tags are assigned
- processReauthTags: clear UserID/User when tags are applied
during re-authentication
- validateNodeOwnership: reject tagged nodes with non-nil UserID
- NodeStore: skip nodesByUser indexing for tagged nodes since
they have no owning user
- HandleNodeFromPreAuthKey: add fallback lookup for tagged PAK
re-registration (tagged nodes indexed under UserID(0)); guard
against nil User deref for tagged nodes in different-user check
Since tagged nodes now have user_id = NULL, ListNodesByUser
will not return them and DestroyUser naturally allows deleting
users whose nodes have all been tagged. The ON DELETE CASCADE
FK cannot reach tagged nodes through a NULL foreign key.
Also tone down shouty comments throughout state.go.
Fixes#3077
75e56df9e4