Add authentication mismatch detection and pgpass support

Phase 1: Detection & Guidance
- Detect OS user vs DB user mismatch
- Identify PostgreSQL authentication method (peer/ident/md5)
- Show helpful error messages with 4 solutions:
  1. sudo -u <user> (for peer auth)
  2. ~/.pgpass file (recommended)
  3. PGPASSWORD env variable
  4. --password flag

Phase 2: pgpass Support
- Auto-load passwords from ~/.pgpass file
- Support standard PostgreSQL pgpass format
- Check file permissions (must be 0600)
- Support wildcard matching (host:port:db:user:pass)

Tested on CentOS Stream 10 with PostgreSQL 16

internal/config/config.go

@@ -387,6 +387,11 @@ func getCurrentUser() string {
 	return "postgres"
 }
 
+// GetCurrentOSUser returns the current OS user (exported for auth checking)
+func GetCurrentOSUser() string {
+	return getCurrentUser()
+}
+
 func getDefaultBackupDir() string {
 	// Try to create a sensible default backup directory
 	homeDir, _ := os.UserHomeDir()