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-proto/Cargo.toml

@@ -11,5 +11,12 @@ build       = "build.rs"
 [dependencies]
 capnp       = { workspace = true }
 
+[lints.rust]
+unsafe_code = "warn"
+
+[lints.clippy]
+# Generated Cap'n Proto code uses patterns that trigger clippy lints.
+unwrap_used = "allow"
+
 [build-dependencies]
 capnpc      = { workspace = true }