feat: add settings cache to reduce database queries in MediaScanner

- Add SettingsCache with TTL-based invalidation (30 seconds)
- Cache scan_poll_interval_seconds and auto_scan_enabled settings
- Reduce database queries from every poll/check to once per TTL period
- Improve error handling with proper fallback values
- Simplify boolean parsing with strings.ToLower for consistency

This optimization reduces database load when checking scan settings,
which occurs frequently during media scanning operations.

internal/services/cache.go

@@ -0,0 +1,57 @@
+package services
+
+import (
+	"sync"
+	"time"
+)
+
+type SettingsCache struct {
+	data       map[string]string
+	mu         sync.RWMutex
+	ttl        time.Duration
+	lastUpdate time.Time
+}
+
+func NewSettingsCache(ttl time.Duration) *SettingsCache {
+	return &SettingsCache{
+		data:       make(map[string]string),
+		ttl:        ttl,
+		lastUpdate: time.Now(),
+	}
+}
+
+func (c *SettingsCache) Get(key string) (string, bool) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	// Check if cache is expired
+	if time.Since(c.lastUpdate) > c.ttl {
+		return "", false
+	}
+
+	val, ok := c.data[key]
+	return val, ok
+}
+
+func (c *SettingsCache) Set(key, value string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.data[key] = value
+	c.lastUpdate = time.Now()
+}
+
+func (c *SettingsCache) Invalidate() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.data = make(map[string]string)
+	c.lastUpdate = time.Time{}
+}
+
+func (c *SettingsCache) InvalidateKey(key string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	delete(c.data, key)
+}