feat: Sprint 1 — production hardening, TLS lifecycle, CI coverage, lint cleanup

- Fix 3 client panics: replace .unwrap()/.expect() with proper error
  handling in rpc.rs (AUTH_CONTEXT lock), repl.rs (pending_member),
  and retry.rs (last_err)
- Add --danger-accept-invalid-certs flag with InsecureServerCertVerifier
  for development TLS bypass, plus mdBook TLS documentation
- Add CI coverage job (cargo-tarpaulin) and Docker build validation
  to GitHub Actions workflow, plus README CI badge
- Add [workspace.lints] config, fix 46 clippy warnings across 8 crates,
  zero warnings on all buildable crates
- Update Dockerfile for all 11 workspace members

README.md

@@ -4,6 +4,8 @@
 
 # QPQ — quicproquo
 
+[![CI](https://github.com/nickvidal/quicproquo/actions/workflows/ci.yml/badge.svg)](https://github.com/nickvidal/quicproquo/actions/workflows/ci.yml)
+
 > End-to-end encrypted messaging over **QUIC + TLS 1.3 + MLS** (RFC 9420), written in Rust.
 
 The server never sees plaintext. Every byte on the wire is protected by a QUIC