chore: add Docker deployment setup and health check wiring

- Add multi-stage Dockerfile for backend (SDK build → ASP.NET runtime,
  non-root user, /health HEALTHCHECK)
- Add docker-compose.dev.yml orchestrating extrudex-api + control-center-web
- Add deploy.sh convenience script wrapping docker compose up --build
- Wire ASP.NET health checks: AddHealthChecks().AddNpgSql() + MapHealthChecks("/health")
- Add backend .dockerignore (comprehensive pattern list)
- Exclude frontend/dist, frontend/node_modules, frontend/.angular from git

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Program.cs

@@ -69,6 +69,10 @@ builder.Services.AddCors(options =>
 // ── SignalR (real-time printer updates) ────────────────────
 builder.Services.AddSignalR();
 
+// ── Health Checks ───────────────────────────────────────────
+builder.Services.AddHealthChecks()
+    .AddNpgSql(connectionString);
+
 var app = builder.Build();
 
 // ── Middleware ──────────────────────────────────────────────
@@ -85,6 +89,9 @@ app.MapControllers();
 // ── Hub Endpoints ───────────────────────────────────────────
 app.MapHub<PrinterHub>("/hubs/printer");
 
+// ── Health Check Endpoint ──────────────────────────────────
+app.MapHealthChecks("/health");
+
 app.Run();
 
 // Helper: builds a connection string from individual env vars.