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>

Dockerfile

@@ -8,6 +8,7 @@ RUN go mod download
 
 COPY . .
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /metacrypt ./cmd/metacrypt
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /metacrypt-web ./cmd/metacrypt-web
 
 FROM alpine:3.21
 
@@ -17,7 +18,7 @@ RUN apk add --no-cache ca-certificates tzdata \
     && mkdir -p /srv/metacrypt && chown metacrypt:metacrypt /srv/metacrypt
 
 COPY --from=builder /metacrypt /usr/local/bin/metacrypt
-COPY web/ /srv/metacrypt/web/
+COPY --from=builder /metacrypt-web /usr/local/bin/metacrypt-web
 
 # /srv/metacrypt is the single volume mount point.
 # It must contain: