Admin endpoints for bans and clients on the single listener (closes #27)

SWWAF_ADMIN_TOKEN, or its _FILE form, opens GET and POST
/_smallwebwaf/bans, DELETE /_smallwebwaf/bans/<client> and GET
/_smallwebwaf/clients/<ip>. Unset, they answer 404; a missing or wrong
token gets 401, in observe mode too. They go through every check, as
the metrics do. POST takes a netblock, not IPv4-mapped and without a
zone, or a client's address, a duration or permanent, and a reason, and
makes an admin ban even while another lasts. DELETE lifts every active
ban covering the address, kept and marked lifted. Bans come back as
bans.json entries; a client as clients.json holds it, with its bans.

Judgement call: answers leave out bans.json's version field.
Judgement call: DELETE takes an address, not a netblock.
Rule suppressed: gosec G304 on a test reading bans.json.

Model: opus-5-5

internal/proxy/proxy.go

@@ -38,6 +38,14 @@ const HealthPath = "/_smallwebwaf/healthz"
 // SWWAF_METRICS_TOKEN.
 const MetricsPath = "/_smallwebwaf/metrics"
 
+// BansPath is where an admin lists and adds bans, and, followed by / and
+// a client's address, lifts them, with SWWAF_ADMIN_TOKEN.
+const BansPath = "/_smallwebwaf/bans"
+
+// ClientsPath is where an admin asks what smallwebwaf knows of a client,
+// by the client's address after it, with SWWAF_ADMIN_TOKEN.
+const ClientsPath = "/_smallwebwaf/clients/"
+
 // Params are what New needs.
 type Params struct {
 	Config *config.Config