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

crates/quicproquo-kt/Cargo.toml

@@ -5,6 +5,9 @@ edition     = "2021"
 description = "Key Transparency: append-only SHA-256 Merkle log for (username, identity_key) bindings."
 license     = "MIT"
 
+[lints]
+workspace = true
+
 [dependencies]
 sha2      = { workspace = true }
 thiserror = { workspace = true }