Implement MCNS v1: custom Go DNS server replacing CoreDNS

Replace the CoreDNS precursor with a purpose-built authoritative DNS
server. Zones and records (A, AAAA, CNAME) are stored in SQLite and
managed via synchronized gRPC + REST APIs authenticated through MCIAS.
Non-authoritative queries are forwarded to upstream resolvers with
in-memory caching.

Key components:
- DNS server (miekg/dns) with authoritative zone handling and forwarding
- gRPC + REST management APIs with MCIAS auth (mcdsl integration)
- SQLite storage with CNAME exclusivity enforcement and auto SOA serials
- 30 tests covering database CRUD, DNS resolution, and caching

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/grpcserver/admin.go

@@ -0,0 +1,20 @@
+package grpcserver
+
+import (
+	"context"
+
+	pb "git.wntrmute.dev/kyle/mcns/gen/mcns/v1"
+	"git.wntrmute.dev/kyle/mcns/internal/db"
+)
+
+type adminService struct {
+	pb.UnimplementedAdminServiceServer
+	db *db.DB
+}
+
+func (s *adminService) Health(_ context.Context, _ *pb.HealthRequest) (*pb.HealthResponse, error) {
+	if err := s.db.Ping(); err != nil {
+		return &pb.HealthResponse{Status: "unhealthy"}, nil
+	}
+	return &pb.HealthResponse{Status: "ok"}, nil
+}