Add reliability improvements and config persistence feature

- Implement context cleanup with sync.Once and io.Closer interface
- Add regex-based error classification for robust error handling
- Create ProcessManager with thread-safe process tracking
- Add disk space caching with 30s TTL for performance
- Implement metrics collection with structured logging
- Add config persistence (.dbbackup.conf) for directory-local settings
- Auto-save/auto-load configuration with --no-config and --no-save-config flags
- Successfully tested with 42GB d7030 database (35K large objects, 36min backup)
- All cross-platform builds working (9/10 platforms)

internal/checks/cache.go

@@ -0,0 +1,83 @@
+package checks
+
+import (
+	"sync"
+	"time"
+)
+
+// cacheEntry holds cached disk space information with TTL
+type cacheEntry struct {
+	check     *DiskSpaceCheck
+	timestamp time.Time
+}
+
+// DiskSpaceCache provides thread-safe caching of disk space checks with TTL
+type DiskSpaceCache struct {
+	cache    map[string]*cacheEntry
+	cacheTTL time.Duration
+	mu       sync.RWMutex
+}
+
+// NewDiskSpaceCache creates a new disk space cache with specified TTL
+func NewDiskSpaceCache(ttl time.Duration) *DiskSpaceCache {
+	if ttl <= 0 {
+		ttl = 30 * time.Second // Default 30 second cache
+	}
+	
+	return &DiskSpaceCache{
+		cache:    make(map[string]*cacheEntry),
+		cacheTTL: ttl,
+	}
+}
+
+// Get retrieves cached disk space check or performs new check if cache miss/expired
+func (c *DiskSpaceCache) Get(path string) *DiskSpaceCheck {
+	c.mu.RLock()
+	if entry, exists := c.cache[path]; exists {
+		if time.Since(entry.timestamp) < c.cacheTTL {
+			c.mu.RUnlock()
+			return entry.check
+		}
+	}
+	c.mu.RUnlock()
+	
+	// Cache miss or expired - perform new check
+	check := CheckDiskSpace(path)
+	
+	c.mu.Lock()
+	c.cache[path] = &cacheEntry{
+		check:     check,
+		timestamp: time.Now(),
+	}
+	c.mu.Unlock()
+	
+	return check
+}
+
+// Clear removes all cached entries
+func (c *DiskSpaceCache) Clear() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.cache = make(map[string]*cacheEntry)
+}
+
+// Cleanup removes expired entries (call periodically)
+func (c *DiskSpaceCache) Cleanup() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	
+	now := time.Now()
+	for path, entry := range c.cache {
+		if now.Sub(entry.timestamp) >= c.cacheTTL {
+			delete(c.cache, path)
+		}
+	}
+}
+
+// Global cache instance with 30-second TTL
+var globalDiskCache = NewDiskSpaceCache(30 * time.Second)
+
+// CheckDiskSpaceCached performs cached disk space check
+func CheckDiskSpaceCached(path string) *DiskSpaceCheck {
+	return globalDiskCache.Get(path)
+}