feat: add resilience and reliability features for agent subsystems

Added circuit breakers with configurable timeouts for all subsystems (APT, DNF, Docker, Windows, Winget, Storage). Replaces cron-based scheduler with priority queue that should scale beyond 1000+ agents if your homelab is that big.

Command acknowledgment system ensures results aren't lost on network failures or restarts. Agent tracks pending acknowledgments with persistent state and automatic retry.

- Circuit breakers: 3 failures in 1min opens circuit, 30s cooldown
- Per-subsystem timeouts: 30s-10min depending on scanner
- Priority queue scheduler: O(log n), worker pool, jitter, backpressure
- Acknowledgments: at-least-once delivery, max 10 retries over 24h
- All tests passing (26/26)

aggregator-server/cmd/server/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"flag"
 	"fmt"
 	"log"
@@ -12,6 +13,7 @@ import (
 	"github.com/Fimeg/RedFlag/aggregator-server/internal/config"
 	"github.com/Fimeg/RedFlag/aggregator-server/internal/database"
 	"github.com/Fimeg/RedFlag/aggregator-server/internal/database/queries"
+	"github.com/Fimeg/RedFlag/aggregator-server/internal/scheduler"
 	"github.com/Fimeg/RedFlag/aggregator-server/internal/services"
 	"github.com/gin-gonic/gin"
 )
@@ -285,10 +287,45 @@ func main() {
 	timeoutService.Start()
 	log.Println("Timeout service started")
 
-	// Add graceful shutdown for timeout service
+	// Initialize and start scheduler
+	schedulerConfig := scheduler.DefaultConfig()
+	subsystemScheduler := scheduler.NewScheduler(schedulerConfig, agentQueries, commandQueries)
+
+	// Load subsystems into queue
+	ctx := context.Background()
+	if err := subsystemScheduler.LoadSubsystems(ctx); err != nil {
+		log.Printf("Warning: Failed to load subsystems: %v", err)
+	} else {
+		log.Println("Subsystems loaded into scheduler")
+	}
+
+	// Start scheduler
+	if err := subsystemScheduler.Start(); err != nil {
+		log.Printf("Warning: Failed to start scheduler: %v", err)
+	}
+
+	// Add scheduler stats endpoint (after scheduler is initialized)
+	router.GET("/api/v1/scheduler/stats", middleware.AuthMiddleware(), func(c *gin.Context) {
+		stats := subsystemScheduler.GetStats()
+		queueStats := subsystemScheduler.GetQueueStats()
+		c.JSON(200, gin.H{
+			"scheduler": stats,
+			"queue":     queueStats,
+		})
+	})
+
+	// Add graceful shutdown for services
 	defer func() {
+		log.Println("Shutting down services...")
+
+		// Stop scheduler first
+		if err := subsystemScheduler.Stop(); err != nil {
+			log.Printf("Error stopping scheduler: %v", err)
+		}
+
+		// Stop timeout service
 		timeoutService.Stop()
-		log.Println("Timeout service stopped")
+		log.Println("Services stopped")
 	}()
 
 	// Start server