feat: add dag_engine app — CLI + web frontend for DAG execution (0007e)

Full DAG engine app with CLI subcommands (run, list, status, validate, server)
and React/Mantine web frontend. Uses net/http + embedded Vite build. SQLite
store for run history. Scheduler with cron_ticker for automated execution.
Compatible with existing dagu YAML format.

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

apps/dag_engine/scheduler.go

@@ -0,0 +1,188 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"fn-registry/functions/core"
+	"fn-registry/functions/infra"
+)
+
+// ScheduledDAG represents a DAG with a parsed cron schedule.
+type ScheduledDAG struct {
+	Name     string    `json:"name"`
+	Path     string    `json:"path"`
+	Schedule string    `json:"schedule"`
+	NextRun  time.Time `json:"next_run"`
+}
+
+// Scheduler manages cron-triggered DAG execution.
+type Scheduler struct {
+	mu       sync.Mutex
+	running  bool
+	cancel   context.CancelFunc
+	dagsDir  string
+	executor *Executor
+	dags     []ScheduledDAG
+}
+
+// NewScheduler creates a new scheduler.
+func NewScheduler(executor *Executor, dagsDir string) *Scheduler {
+	return &Scheduler{
+		executor: executor,
+		dagsDir:  dagsDir,
+	}
+}
+
+// Start scans for DAGs with schedules and starts cron tickers for each.
+func (s *Scheduler) Start() error {
+	s.mu.Lock()
+	if s.running {
+		s.mu.Unlock()
+		return fmt.Errorf("scheduler already running")
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	s.cancel = cancel
+	s.running = true
+	s.mu.Unlock()
+
+	scheduled, err := s.scanDAGs()
+	if err != nil {
+		s.mu.Lock()
+		s.running = false
+		s.mu.Unlock()
+		cancel()
+		return err
+	}
+
+	s.mu.Lock()
+	s.dags = scheduled
+	s.mu.Unlock()
+
+	log.Printf("[scheduler] started with %d DAGs", len(scheduled))
+
+	for _, dag := range scheduled {
+		dag := dag
+		go s.runTicker(ctx, dag)
+	}
+
+	return nil
+}
+
+// Stop cancels all tickers and stops the scheduler.
+func (s *Scheduler) Stop() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if !s.running {
+		return
+	}
+	s.cancel()
+	s.running = false
+	s.dags = nil
+	log.Printf("[scheduler] stopped")
+}
+
+// IsRunning returns true if the scheduler is active.
+func (s *Scheduler) IsRunning() bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.running
+}
+
+// Status returns the list of scheduled DAGs with their next run time.
+type SchedulerStatus struct {
+	Running bool           `json:"running"`
+	DAGs    []ScheduledDAG `json:"dags"`
+}
+
+func (s *Scheduler) Status() SchedulerStatus {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return SchedulerStatus{
+		Running: s.running,
+		DAGs:    s.dags,
+	}
+}
+
+// scanDAGs reads the dags directory and returns DAGs that have cron schedules.
+func (s *Scheduler) scanDAGs() ([]ScheduledDAG, error) {
+	entries, err := os.ReadDir(s.dagsDir)
+	if err != nil {
+		return nil, err
+	}
+
+	var scheduled []ScheduledDAG
+	for _, entry := range entries {
+		ext := filepath.Ext(entry.Name())
+		if ext != ".yaml" && ext != ".yml" {
+			continue
+		}
+
+		path := filepath.Join(s.dagsDir, entry.Name())
+		data, err := os.ReadFile(path)
+		if err != nil {
+			continue
+		}
+
+		dag, err := core.DagParse(data)
+		if err != nil {
+			continue
+		}
+
+		for _, expr := range dag.Schedule {
+			sched, err := core.ParseCronExpr(strings.TrimSpace(expr))
+			if err != nil {
+				log.Printf("[scheduler] invalid cron %q in %s: %v", expr, dag.Name, err)
+				continue
+			}
+			next := core.NextCronTime(sched, time.Now())
+			scheduled = append(scheduled, ScheduledDAG{
+				Name:     dag.Name,
+				Path:     path,
+				Schedule: expr,
+				NextRun:  next,
+			})
+		}
+	}
+
+	return scheduled, nil
+}
+
+// runTicker starts a cron ticker for a single DAG schedule.
+func (s *Scheduler) runTicker(ctx context.Context, dag ScheduledDAG) {
+	sched, err := core.ParseCronExpr(strings.TrimSpace(dag.Schedule))
+	if err != nil {
+		return
+	}
+
+	// Convert core.CronSchedule to infra.CronTickerSchedule.
+	tickerSched := infra.CronTickerSchedule{
+		Minute:     sched.Minute,
+		Hour:       sched.Hour,
+		DayOfMonth: sched.DayOfMonth,
+		Month:      sched.Month,
+		DayOfWeek:  sched.DayOfWeek,
+	}
+
+	ch := infra.CronTicker(tickerSched, ctx)
+	log.Printf("[scheduler] ticker started for %s (%s), next: %s", dag.Name, dag.Schedule, dag.NextRun.Format(time.RFC3339))
+
+	for t := range ch {
+		log.Printf("[scheduler] triggered %s at %s", dag.Name, t.Format(time.RFC3339))
+		go func() {
+			runID, err := s.executor.ExecuteDAG(ctx, dag.Path, "cron")
+			if err != nil {
+				log.Printf("[scheduler] %s failed: %v (run: %s)", dag.Name, err, runID)
+			} else {
+				log.Printf("[scheduler] %s completed (run: %s)", dag.Name, runID)
+			}
+		}()
+	}
+}