feat(membership): bound request bodies and add per-IP rate limit

Pre-auth DoS hardening (audit H1, Critical). The control-plane middleware
read the request body with io.ReadAll before authenticating and with no size
cap, so an unauthenticated peer could force the server to buffer an arbitrary
body in RAM (the auditor sent 400 MB and watched RSS climb to ~898 MB).

- ServeHTTP now caps the buffered body before reading: a per-route ceiling
  (1 MiB JSON, 16 MiB /blobs) rejects an over-declared Content-Length outright
  and wraps the body in http.MaxBytesReader so a lying/chunked sender trips at
  the ceiling instead of unbounded.
- handlePutBlob maps the MaxBytesReader cutoff to 413 in every auth mode.
- Per-IP token-bucket rate limiter (golang.org/x/time/rate, already in the
  module graph) sheds floods before auth or body reads. Loopback dev stacks are
  unaffected (burst >> any single client's rate). Kept in-package as transport
  glue, not promoted to the registry, mirroring the nonceCache decision in 0003.
- membershipd sets http.Server.MaxHeaderBytes and ReadHeaderTimeout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.sum

@@ -1,5 +1,7 @@
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@@ -47,6 +49,10 @@ golang.org/x/time v0.7.0 h1:ntUhktv3OPE6TgYxXWv9vKvUSJyIFJlyohwbkEwPrKQ=
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