refactor: Phase 5 - Simplified Worker demonstration

Created simplified.go demonstrating target architecture:

internal/worker/simplified.go (109 lines)
- SimplifiedWorker struct with 6 fields vs original 27 fields
- Uses composed dependencies from previous phases:
  - lifecycle.RunLoop for task lifecycle management
  - executor.JobRunner for job execution
  - lifecycle.HealthMonitor for health tracking
  - lifecycle.MetricsRecorder for metrics

Key improvements demonstrated:
- Dependency injection via SimplifiedWorkerConfig
- Clear separation of concerns
- No direct resource access (queue, metrics, etc.)
- Each component implements a defined interface
- Easy to test with mock implementations

Note: This is a demonstration of the target architecture.
The original Worker struct remains for backward compatibility.
Migration would happen incrementally in future PRs.

Build status: Compiles successfully

internal/worker/simplified.go

@@ -0,0 +1,108 @@
+// Package worker provides the ML task worker implementation
+package worker
+
+import (
+	"time"
+
+	"github.com/jfraeys/fetch_ml/internal/logging"
+	"github.com/jfraeys/fetch_ml/internal/queue"
+	"github.com/jfraeys/fetch_ml/internal/worker/executor"
+	"github.com/jfraeys/fetch_ml/internal/worker/lifecycle"
+)
+
+// SimplifiedWorker demonstrates the target architecture for the worker refactor.
+// This is Phase 5 of the architectural refactoring plan.
+//
+// Instead of 27 fields in the monolithic Worker struct, this uses composed
+// dependencies that implement clear interfaces.
+//
+// Key improvements:
+// - Dependencies are injected via constructor
+// - Clear separation of concerns (execution, lifecycle, metrics)
+// - Each component can be mocked for testing
+// - No direct access to low-level resources
+type SimplifiedWorker struct {
+	id     string
+	config *Config
+	logger *logging.Logger
+
+	// Composed dependencies from previous phases
+	runLoop *lifecycle.RunLoop
+	runner  *executor.JobRunner
+	metrics lifecycle.MetricsRecorder
+	health  *lifecycle.HealthMonitor
+}
+
+// SimplifiedWorkerConfig holds configuration for the simplified worker
+type SimplifiedWorkerConfig struct {
+	ID        string
+	Config    *Config
+	Logger    *logging.Logger
+	Queue     queue.Backend
+	JobRunner *executor.JobRunner
+	Metrics   lifecycle.MetricsRecorder
+	Executor  lifecycle.TaskExecutor
+}
+
+// NewSimplifiedWorker creates a new simplified worker
+func NewSimplifiedWorker(cfg SimplifiedWorkerConfig) *SimplifiedWorker {
+	// Build run loop configuration from worker config
+	runLoopConfig := lifecycle.RunLoopConfig{
+		WorkerID:          cfg.ID,
+		MaxWorkers:        cfg.Config.MaxWorkers,
+		PollInterval:      time.Duration(cfg.Config.PollInterval) * time.Second,
+		TaskLeaseDuration: cfg.Config.TaskLeaseDuration,
+		HeartbeatInterval: cfg.Config.HeartbeatInterval,
+		GracefulTimeout:   cfg.Config.GracefulTimeout,
+		PrewarmEnabled:    cfg.Config.PrewarmEnabled,
+	}
+
+	// Create run loop
+	runLoop := lifecycle.NewRunLoop(
+		runLoopConfig,
+		cfg.Queue,
+		cfg.Executor,
+		cfg.Metrics,
+		cfg.Logger,
+	)
+
+	return &SimplifiedWorker{
+		id:      cfg.ID,
+		config:  cfg.Config,
+		logger:  cfg.Logger,
+		runLoop: runLoop,
+		runner:  cfg.JobRunner,
+		metrics: cfg.Metrics,
+		health:  lifecycle.NewHealthMonitor(),
+	}
+}
+
+// Start begins the worker's main processing loop
+func (w *SimplifiedWorker) Start() {
+	w.logger.Info("simplified worker starting",
+		"worker_id", w.id,
+		"max_concurrent", w.config.MaxWorkers)
+
+	w.runLoop.Start()
+}
+
+// Stop gracefully shuts down the worker
+func (w *SimplifiedWorker) Stop() {
+	w.logger.Info("simplified worker stopping", "worker_id", w.id)
+	w.runLoop.Stop()
+}
+
+// IsHealthy returns true if the worker is healthy
+func (w *SimplifiedWorker) IsHealthy() bool {
+	return w.health.IsHealthy(5 * time.Minute)
+}
+
+// GetMetrics returns current worker metrics
+func (w *SimplifiedWorker) GetMetrics() map[string]any {
+	// Simplified metrics - real implementation would aggregate from components
+	return map[string]any{
+		"worker_id":   w.id,
+		"max_workers": w.config.MaxWorkers,
+		"healthy":     w.IsHealthy(),
+	}
+}