feat(telemetry): add caching, alerts, migration & dashboard improvements

- Add Redis/in-memory caching layer (cache.go)
- Add SMTP alerting for high failure rates (alerts.go)
- Add data migration script from old API (migrate.go)
- Add docker-compose.yml for easy deployment
- Move dashboard to / with redirect from /dashboard
- Add dark/light mode toggle
- Add error analysis and failed apps statistics
- Add PVE version and LXC/VM type stats
- Add /metrics Prometheus endpoint
- Add /api/records pagination endpoint
- Add CSV export functionality
- Enhanced healthcheck with PB connection status

New ENV vars:
- Cache: ENABLE_CACHE, CACHE_TTL_SECONDS, ENABLE_REDIS, REDIS_URL
- Alerts: ALERT_ENABLED, SMTP_*, ALERT_FAILURE_THRESHOLD, etc.
- Migration: RUN_MIGRATION, MIGRATION_REQUIRED, MIGRATION_SOURCE_URL

misc/data/cache.go

@@ -0,0 +1,158 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"log"
+	"sync"
+	"time"
+
+	"github.com/redis/go-redis/v9"
+)
+
+// CacheConfig holds cache configuration
+type CacheConfig struct {
+	RedisURL    string
+	EnableRedis bool
+	DefaultTTL  time.Duration
+}
+
+// Cache provides caching functionality with Redis or in-memory fallback
+type Cache struct {
+	redis      *redis.Client
+	useRedis   bool
+	defaultTTL time.Duration
+
+	// In-memory fallback
+	mu      sync.RWMutex
+	memData map[string]cacheEntry
+}
+
+type cacheEntry struct {
+	data      []byte
+	expiresAt time.Time
+}
+
+// NewCache creates a new cache instance
+func NewCache(cfg CacheConfig) *Cache {
+	c := &Cache{
+		defaultTTL: cfg.DefaultTTL,
+		memData:    make(map[string]cacheEntry),
+	}
+
+	if cfg.EnableRedis && cfg.RedisURL != "" {
+		opts, err := redis.ParseURL(cfg.RedisURL)
+		if err != nil {
+			log.Printf("WARN: invalid redis URL, using in-memory cache: %v", err)
+			return c
+		}
+
+		client := redis.NewClient(opts)
+		ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+		defer cancel()
+
+		if err := client.Ping(ctx).Err(); err != nil {
+			log.Printf("WARN: redis connection failed, using in-memory cache: %v", err)
+			return c
+		}
+
+		c.redis = client
+		c.useRedis = true
+		log.Printf("INFO: connected to Redis for caching")
+	}
+
+	// Start cleanup goroutine for in-memory cache
+	if !c.useRedis {
+		go c.cleanupLoop()
+	}
+
+	return c
+}
+
+func (c *Cache) cleanupLoop() {
+	ticker := time.NewTicker(5 * time.Minute)
+	defer ticker.Stop()
+
+	for range ticker.C {
+		c.mu.Lock()
+		now := time.Now()
+		for k, v := range c.memData {
+			if now.After(v.expiresAt) {
+				delete(c.memData, k)
+			}
+		}
+		c.mu.Unlock()
+	}
+}
+
+// Get retrieves a value from cache
+func (c *Cache) Get(ctx context.Context, key string, dest interface{}) bool {
+	if c.useRedis {
+		data, err := c.redis.Get(ctx, key).Bytes()
+		if err != nil {
+			return false
+		}
+		return json.Unmarshal(data, dest) == nil
+	}
+
+	// In-memory fallback
+	c.mu.RLock()
+	entry, ok := c.memData[key]
+	c.mu.RUnlock()
+
+	if !ok || time.Now().After(entry.expiresAt) {
+		return false
+	}
+
+	return json.Unmarshal(entry.data, dest) == nil
+}
+
+// Set stores a value in cache
+func (c *Cache) Set(ctx context.Context, key string, value interface{}, ttl time.Duration) error {
+	if ttl == 0 {
+		ttl = c.defaultTTL
+	}
+
+	data, err := json.Marshal(value)
+	if err != nil {
+		return err
+	}
+
+	if c.useRedis {
+		return c.redis.Set(ctx, key, data, ttl).Err()
+	}
+
+	// In-memory fallback
+	c.mu.Lock()
+	c.memData[key] = cacheEntry{
+		data:      data,
+		expiresAt: time.Now().Add(ttl),
+	}
+	c.mu.Unlock()
+
+	return nil
+}
+
+// Delete removes a key from cache
+func (c *Cache) Delete(ctx context.Context, key string) error {
+	if c.useRedis {
+		return c.redis.Del(ctx, key).Err()
+	}
+
+	c.mu.Lock()
+	delete(c.memData, key)
+	c.mu.Unlock()
+	return nil
+}
+
+// InvalidateDashboard clears dashboard cache
+func (c *Cache) InvalidateDashboard(ctx context.Context) {
+	// Delete all dashboard cache keys
+	for days := 1; days <= 365; days++ {
+		_ = c.Delete(ctx, dashboardCacheKey(days))
+	}
+}
+
+func dashboardCacheKey(days int) string {
+	return "dashboard:" + string(rune(days))
+}