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

Adding or editing an http or slack target whose address is private or
reserved was refused with no hint that the refusal is deliberate or
that it can be lifted. The refusal now adds that such addresses are
refused by default and that the server's ALLOWED_EGRESS_CIDRS setting
allows named networks, naming the README section "Allowing egress to
your own network". Metadata refusals do not get it.

The default blocklist's public addresses move to a list of their own,
still checked after the allowlist, and are refused as cloud metadata
addresses. The private-and-reserved error is exported as
ErrBlockedPrivateOrReservedIP so the handler can tell them apart.

Model: opus-5-5

internal/handlers/source_management.go

@@ -1578,11 +1578,22 @@ func (h *Handlers) validateTargetURL(
 			"url", delivery.MaskURL(targetURL),
 			"error", err,
 		)
-		http.Error(
-			w,
-			"Invalid target URL: "+err.Error(),
-			http.StatusBadRequest,
-		)
+
+		msg := "Invalid target URL: " + err.Error()
+
+		// Only a private or reserved address's refusal says how
+		// to allow it. Metadata refusals never do: link-local and
+		// the other unconditional metadata addresses cannot be
+		// opened, and the default blocklist's public addresses,
+		// which listing does open, hand out credentials.
+		if errors.Is(err, delivery.ErrBlockedPrivateOrReservedIP) {
+			msg += ". Private and reserved addresses are refused " +
+				"by default; the server's ALLOWED_EGRESS_CIDRS " +
+				"setting allows named networks (see \"Allowing " +
+				"egress to your own network\" in the README)."
+		}
+
+		http.Error(w, msg, http.StatusBadRequest)
 
 		return err
 	}