findings of the security review of stages 2-6

The review of 4433688..0808779 (read-only, a separate agent) found one
medium and some lows; all fixed here.

Medium, proven by the reviewer: A1's userKey put the typed username into
the limiter map whole, and the public forms had no body bound beyond
net/http's 10 MiB for a parsed form - each refused request kept up to
10 MiB in memory for fifteen minutes. userKey now holds a 96-bit hash of
the folded name, and sign-in, reset, a reset link's page and setup read at
most 64 KiB (limitForm, http.MaxBytesReader).
TestPublicFormsAreBoundedAndKeysShort.

Lows:
- Refund skipped the request that filled a bucket (count < max), so a
  right password at the ceiling left the shared address blocked; Refund
  now always undoes the Allow it follows.
- The reset form did not give the address its attempt back when the
  name's bucket refused (sign-in did); it does.
- A3's day of five links is spent only by a link that goes out, and the
  price of the pause - anyone knowing a username can pause that owner's
  reset for a day - is recorded in security.md.
- longHeaderLine counted a trailing CR on a header's first line.
- test.yml declares permissions: contents: read.

Not changed, as the review's nits: the store matches names exactly while
the limiter folds them (stricter, said in the comment now); a session from
a longer window is honoured until its next Touch (the next request).

go test ./internal/web/auth ./internal/milter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/auth/handlers.go

@@ -115,6 +115,7 @@ func (m *Module) submitLogin(w http.ResponseWriter, r *http.Request) {
 		m.renderLogin(w, http.StatusTooManyRequests, "Too many attempts. Please wait and try again.")
 		return
 	}
+	limitForm(w, r)
 	if err := r.ParseForm(); err != nil {
 		m.renderLogin(w, http.StatusBadRequest, "Invalid form submission.")
 		return
@@ -250,6 +251,7 @@ func (m *Module) renderSetupForm(w http.ResponseWriter, status int, token, formE
 }
 
 func (m *Module) submitSetup(w http.ResponseWriter, r *http.Request, token string) {
+	limitForm(w, r)
 	if err := r.ParseForm(); err != nil {
 		m.renderSetupForm(w, http.StatusBadRequest, token, "Invalid form submission.")
 		return