Separate web UI into standalone metacrypt-web binary

The vault server holds in-memory unsealed state (KEK, engine keys) that
is lost on restart, requiring a full unseal ceremony. Previously the web
UI ran inside the vault process, so any UI change forced a restart and
re-unseal.

This change extracts the web UI into a separate metacrypt-web binary
that communicates with the vault over an authenticated gRPC connection.
The web server carries no sealed state and can be restarted freely.

- gen/metacrypt/v1/: generated Go bindings from proto/metacrypt/v1/
- internal/grpcserver/: full gRPC server implementation (System, Auth,
  Engine, PKI, Policy, ACME services) with seal/auth/admin interceptors
- internal/webserver/: web server with gRPC vault client; templates
  embedded via web/embed.go (no runtime web/ directory needed)
- cmd/metacrypt-web/: standalone binary entry point
- internal/config: added [web] section (listen_addr, vault_grpc, etc.)
- internal/server/routes.go: removed all web UI routes and handlers
- cmd/metacrypt/server.go: starts gRPC server alongside HTTP server
- Deploy: Dockerfile builds both binaries, docker-compose adds
  metacrypt-web service, new metacrypt-web.service systemd unit,
  Makefile gains proto/metacrypt-web targets

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deploy/systemd/metacrypt-web.service

@@ -0,0 +1,46 @@
+[Unit]
+Description=Metacrypt web UI server
+Documentation=https://git.wntrmute.dev/kyle/metacrypt
+After=network-online.target metacrypt.service
+Wants=network-online.target
+Requires=metacrypt.service
+
+[Service]
+Type=simple
+User=metacrypt
+Group=metacrypt
+
+ExecStart=/usr/local/bin/metacrypt-web --config /srv/metacrypt/metacrypt.toml
+ExecReload=/bin/kill -HUP $MAINPID
+
+Restart=on-failure
+RestartSec=5
+
+# Security hardening
+NoNewPrivileges=true
+ProtectSystem=strict
+ProtectHome=true
+PrivateTmp=true
+PrivateDevices=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectControlGroups=true
+RestrictSUIDSGID=true
+RestrictNamespaces=true
+LockPersonality=true
+MemoryDenyWriteExecute=true
+RestrictRealtime=true
+
+# Allow read access to config and certs
+ReadOnlyPaths=/srv/metacrypt
+
+# Limit file descriptor count
+LimitNOFILE=65535
+
+# Logging
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=metacrypt-web
+
+[Install]
+WantedBy=multi-user.target