Implement Phase 7: gRPC dual-stack interface

- proto/mcias/v1/: AdminService, AuthService, TokenService,
  AccountService, CredentialService; generated Go stubs in gen/
- internal/grpcserver: full handler implementations sharing all
  business logic (auth, token, db, crypto) with REST server;
  interceptor chain: logging -> auth (JWT alg-first + revocation) ->
  rate-limit (token bucket, 10 req/s, burst 10, per-IP)
- internal/config: optional grpc_addr field in [server] section
- cmd/mciassrv: dual-stack startup; gRPC/TLS listener on grpc_addr
  when configured; graceful shutdown of both servers in 15s window
- cmd/mciasgrpcctl: companion gRPC CLI mirroring mciasctl commands
  (health, pubkey, account, role, token, pgcreds) using TLS with
  optional custom CA cert
- internal/grpcserver/grpcserver_test.go: 20 tests via bufconn covering
  public RPCs, auth interceptor (no token, invalid, revoked -> 401),
  non-admin -> 403, Login/Logout/RenewToken/ValidateToken flows,
  AccountService CRUD, SetPGCreds/GetPGCreds AES-GCM round-trip,
  credential fields absent from all responses
Security:
  JWT validation path identical to REST: alg header checked before
  signature, alg:none rejected, revocation table checked after sig.
  Authorization metadata value never logged by any interceptor.
  Credential fields (PasswordHash, TOTPSecret*, PGPassword) absent from
  all proto response messages — enforced by proto design and confirmed
  by test TestCredentialFieldsAbsentFromAccountResponse.
  Login dummy-Argon2 timing guard preserves timing uniformity for
  unknown users (same as REST handleLogin).
  TLS required at listener level; cmd/mciassrv uses
  credentials.NewServerTLSFromFile; no h2c offered.
137 tests pass, zero race conditions (go test -race ./...)

proto/generate.go

@@ -0,0 +1,10 @@
+// Package proto contains the protobuf source definitions for MCIAS.
+//
+// To regenerate Go stubs after editing .proto files:
+//
+//	go generate ./proto/...
+//
+// Prerequisites: protoc, protoc-gen-go, protoc-gen-go-grpc must be in PATH.
+//
+//go:generate protoc --proto_path=../proto --go_out=../gen --go_opt=paths=source_relative --go-grpc_out=../gen --go-grpc_opt=paths=source_relative mcias/v1/common.proto mcias/v1/admin.proto mcias/v1/auth.proto mcias/v1/token.proto mcias/v1/account.proto
+package proto