fix(backend): cut request log fields to the log bound (closes #60)

The request log wrote the URL, User-Agent, Referer and other
request-supplied strings with no length limit, and the server accepts
headers up to 1 MiB, so one request could put about 1 MiB per field
into a log line. Every string the request log takes from the request,
including the request ID chi copies from X-Request-Id, is now cut to
the 128-byte bound the report handler already used. That bound and
its helper moved from the handlers package to the logger package so
both use the one copy.

Model: opus-5-5

backend/internal/logger/logger.go

@@ -11,6 +11,21 @@ import (
 	"go.uber.org/fx"
 )
 
+// MaxLoggedFieldBytes bounds untrusted text (request fields,
+// header values, decode error text) before it is logged, so a
+// caller cannot inflate log volume with an oversized value.
+const MaxLoggedFieldBytes = 128
+
+// BoundedForLog truncates an untrusted string to a fixed byte
+// bound so an attacker-controlled field cannot dominate the log.
+func BoundedForLog(s string) string {
+	if len(s) > MaxLoggedFieldBytes {
+		return s[:MaxLoggedFieldBytes]
+	}
+
+	return s
+}
+
 // Params defines the dependencies for Logger.
 type Params struct {
 	fx.In