Say how to allow a refused private target address (closes #398)

Refusing an http or slack target whose address is private or reserved, on add or edit, now adds one sentence: such addresses are refused by default, and the server's ALLOWED_EGRESS_CIDRS setting allows named networks, with a pointer to the README section. It suggests no value, so it never points at allowing everything.

Metadata refusals get no such sentence. To tell them apart, the default blocklist's public addresses now have their own list, blockedPublicNetworks, still checked after the allowlist; a test pins which addresses each list refuses and how listing opens them, unchanged from before.

Model: opus-5-5

internal/delivery/export_test.go

@@ -40,11 +40,6 @@ const (
 	ExportPendingSweepMinAge = pendingSweepMinAge
 )
 
-// ExportIsBlockedIP exposes isBlockedIP for testing.
-func ExportIsBlockedIP(ip net.IP) bool {
-	return isBlockedIP(ip)
-}
-
 // NewTestGuard builds an SSRF Guard from an explicit egress
 // allowlist, without going through config. Passing no prefixes
 // yields the default guard, which blocks every private/reserved
@@ -70,6 +65,11 @@ func ExportBlockedNetworks() []*net.IPNet {
 	return blockedNetworks
 }
 
+// ExportBlockedPublicNetworks exposes blockedPublicNetworks.
+func ExportBlockedPublicNetworks() []*net.IPNet {
+	return blockedPublicNetworks
+}
+
 // ExportIsForwardableHeader exposes isForwardableHeader.
 func ExportIsForwardableHeader(name string) bool {
 	return isForwardableHeader(name)