Harden server; add authenticated Streamable HTTP transport and Nix module

- Streamable HTTP transport (MCP_TRANSPORT=http) with mandatory auth:
  static bearer tokens or OIDC resource-server mode (JWT via JWKS, or
  RFC 7662 introspection), RFC 9728 protected-resource metadata, and
  DNS-rebinding protection
- secrets loadable from files via *_FILE variants
- timeouts on all outbound requests; JORTT_READ_ONLY tool filtering
- OpenAPI spec committed and bundled; no implicit network fetches
- Nix flake: sandboxed package build + hardened NixOS module taking an
  EnvironmentFile (agenix-friendly)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -3,6 +3,4 @@ dist/
 *.log
 .env
 .DS_Store
-
-# Fetched at build time by scripts/fetch-spec.mjs
-src/openapi.json
+result