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)

TODO.md

@@ -29,6 +29,14 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Completed Steps
 
+- 2026-09-21 http.Server hardening (closes #92): added
+  `HTTPReadHeaderTimeout` (10s, bounds the slowloris header dribble) and
+  `HTTPIdleTimeout` (120s, bounds keep-alive reuse) alongside the
+  existing timeouts and wired them onto the server; added a `LimitBody`
+  middleware capping the two form POST bodies (`POST /`, `POST /generate`)
+  at `MaxFormBytes` (1 MiB) and returning 413, applied ahead of the CSRF
+  middleware so an oversized body is refused as 413 rather than being read
+  as a missing CSRF token (403); left `WriteTimeout` at 60s unchanged
 - 2026-08-07 update golangci-lint to v2.12.2 with the canonical
   `.golangci.yml` (v2 schema, `default: all` minus six disabled
   linters, `lll` 88, tests included): bumped the pinned