Harden http.Server: slowloris timeouts and form body limit (closes #92)

The http.Server now sets ReadHeaderTimeout (10s), which bounds the slow header dribble that ReadTimeout alone does not, and IdleTimeout (120s), which bounds keep-alive reuse. Server construction moved into a small helper so a test can assert the timeouts without binding a listener.

POST / and POST /generate bodies are capped at 1 MiB and an oversized body returns 413.

What a reader would trip over: the CSRF library reads its token from the form and swallows a parse error, so a cap applied only inside it would surface as 403. The body limit therefore parses the form under the cap before the CSRF check; the parsed form is reused afterwards. A test covers an oversized body that carries a valid token.

Judgement call: WriteTimeout stays at 60s; it also bounds how long a large image may take to send over a slow link.

Model: opus-4-8 (implementation, review); fable-5-1 (landing message)

internal/handlers/bodylimit.go

@@ -0,0 +1,45 @@
+package handlers
+
+import (
+	"errors"
+	"net/http"
+)
+
+// MaxFormBytes bounds the request body accepted on the HTML form POST
+// routes (POST / and POST /generate). The forms carry a handful of short
+// fields, so 1 MiB is generous while making the bound explicit rather than
+// resting on ParseForm's incidental 10 MB cap.
+const MaxFormBytes = 1 << 20 // 1 MiB
+
+// LimitBody returns middleware that caps the request body on POST requests
+// at maxBytes and rejects an oversized body with 413 Request Entity Too
+// Large.
+//
+// It parses the form here, before the CSRF middleware reads the token from
+// it. The CSRF middleware reads the token with PostFormValue, which
+// swallows a parse error, so if the body were only capped there an
+// oversized body would read as a missing token and be refused as 403. By
+// parsing under the cap first, an oversized body is refused as 413. A
+// successful parse is cached on the request, so the CSRF check and the
+// handler reuse it rather than reading the body again.
+func (s *Handlers) LimitBody(maxBytes int64) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.Method == http.MethodPost {
+				r.Body = http.MaxBytesReader(w, r.Body, maxBytes)
+
+				err := r.ParseForm()
+
+				var tooLarge *http.MaxBytesError
+				if errors.As(err, &tooLarge) {
+					http.Error(w, "Request body too large",
+						http.StatusRequestEntityTooLarge)
+
+					return
+				}
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}