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/docker/docker-compose.yml

@@ -7,12 +7,9 @@ services:
     restart: unless-stopped
     ports:
       - "8443:8443"
+      - "9443:9443"
     volumes:
       - metacrypt-data:/srv/metacrypt
-    # To populate /srv/metacrypt before first run, use an init container or
-    # bind-mount a host directory instead of a named volume:
-    #   volumes:
-    #     - ./data:/srv/metacrypt
     healthcheck:
       test: ["CMD", "metacrypt", "status", "--addr", "https://localhost:8443", "--ca-cert", "/srv/metacrypt/certs/ca.crt"]
       interval: 30s
@@ -20,5 +17,19 @@ services:
       retries: 3
       start_period: 10s
 
+  metacrypt-web:
+    build:
+      context: ../..
+      dockerfile: Dockerfile
+    container_name: metacrypt-web
+    command: ["/usr/local/bin/metacrypt-web", "--config", "/srv/metacrypt/metacrypt.toml"]
+    restart: unless-stopped
+    ports:
+      - "8080:8080"
+    volumes:
+      - metacrypt-data:/srv/metacrypt
+    depends_on:
+      - metacrypt
+
 volumes:
   metacrypt-data: