Different blocking for different devices: steps and client groups

Client groups put devices (addresses, networks, or ACLs of them) at a
blocking step: off (only your rules), normal (the default), or steps
added above it, each including the lists of the steps below. Lists
start at a step. In each view, a gate policy zone in front of a step's
lists lets the devices below that step through (rpz-client-ip triggers
with rpz-passthru), so every list is still loaded once. The IPv6 part
of a gate leaves the IPv4-mapped range out: named matches IPv4 clients
as ::ffff:a.b.c.d.

Web UI: a Devices panel on the Blocking page and a step on each list.
CLI: jarl block step, jarl block group, jarl blocklist --step. API:
/blocking/steps, /blocking/groups. Database schema 8.

Also: copying the state kept empty lists as empty (not nil), so
reverting a changeset no longer adds restore operations for unchanged
blocklists, rules and views.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/blocking/zone.go

@@ -40,6 +40,9 @@ type Match struct {
 	Entry  string `json:"entry,omitempty"` // the matching entry: the name, or *.parent
 	RuleID int64  `json:"rule_id,omitempty"`
 	List   string `json:"list,omitempty"` // the blocklist, when a list matched
+	// Step is the list's step when it is above normal: only clients at
+	// that step or above get this answer (others: none).
+	Step string `json:"step,omitempty"`
 }
 
 // Actions of rules.