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/ratelimit.go

@@ -75,12 +75,14 @@ func (r *rateLimiter) Allow(key string) bool {
 }
 
 // Refund gives back the attempt Allow took for key, for an attempt that
-// turned out not to be a guess (a right password). A bucket already at its
-// limit stays there: a refund never unblocks.
+// turned out not to be a guess (a right password). It is called only after
+// an Allow that admitted the attempt, so it undoes that one: the request that
+// filled the bucket gives its own place back too (the review of stages 2-6:
+// a right password at the ceiling used to leave the address blocked).
 func (r *rateLimiter) Refund(key string) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	if b := r.buckets[key]; b != nil && b.count > 0 && b.count < r.max {
+	if b := r.buckets[key]; b != nil && b.count > 0 {
 		b.count--
 	}
 }