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/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
 	}