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>

go.mod

@@ -15,8 +15,8 @@ require (
 	github.com/spf13/viper v1.21.0
 	golang.org/x/crypto v0.49.0
 	golang.org/x/term v0.41.0
-	google.golang.org/grpc v1.71.1
-	google.golang.org/protobuf v1.36.5
+	google.golang.org/grpc v1.79.2
+	google.golang.org/protobuf v1.36.11
 	modernc.org/sqlite v1.46.1
 )
 
@@ -37,10 +37,10 @@ require (
 	github.com/subosito/gotenv v1.6.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 // indirect
-	golang.org/x/net v0.40.0 // indirect
+	golang.org/x/net v0.51.0 // indirect
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.35.0 // indirect
-	google.golang.org/genproto/googleapis/rpc v0.0.0-20250303144028-a0af3efb3deb // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
 	modernc.org/libc v1.67.6 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect