fix(backend): Fix Dockerfile and add health check endpoint for Docker

This commit fixes the backend Docker configuration to enable one-click
backend startup for frontend developers.

Changes:
- Updated Dockerfile with correct paths for modular monolith architecture
  * Added all module projects (Identity, ProjectManagement, IssueManagement)
  * Optimized layer caching by copying .csproj files first
  * Used alpine runtime image for smaller size (~500MB reduction)
  * Added non-root user (appuser) for security
  * Simplified to single HTTP port (8080) for development
- Enhanced .dockerignore to optimize build context
  * Excluded unnecessary files (docs, git, docker files)
  * Added environment and secret file exclusions
- Added /health endpoint to Program.cs
  * Required for Docker HEALTHCHECK functionality
  * Enables docker-compose to verify backend is ready

Testing:
- Docker build succeeds in ~14 seconds (after first build)
- Backend container starts and passes health check
- Swagger UI accessible at http://localhost:5000/scalar/v1
- Health endpoint returns "Healthy" at http://localhost:5000/health

This implements Phase 1 of DOCKER-DEVELOPMENT-ENVIRONMENT.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

colaflow-api/src/ColaFlow.API/Program.cs

@@ -156,6 +156,9 @@ builder.Services.AddScoped<ColaFlow.Modules.ProjectManagement.Application.Servic
 // Configure OpenAPI/Scalar
 builder.Services.AddOpenApi();
 
+// Add Health Checks (required for Docker health check)
+builder.Services.AddHealthChecks();
+
 var app = builder.Build();
 
 // Configure the HTTP request pipeline
@@ -188,6 +191,9 @@ app.UseAuthorization();
 
 app.MapControllers();
 
+// Map Health Check endpoint (required for Docker health check)
+app.MapHealthChecks("/health");
+
 // Map SignalR Hubs (after UseAuthorization)
 app.MapHub<ProjectHub>("/hubs/project");
 app.MapHub<NotificationHub>("/hubs/notification");