feat: add IMAP and POP3 plugins for mail server detection

IMAP (143/993): banner grab + LOGIN brute force
POP3 (110/995): banner grab + USER/PASS brute force
Both registered as safe default plugins with auth-check capability.

pkg/fscan/scanner.go

@@ -39,7 +39,9 @@ var defaultSafePlugins = []string{
 	"rabbitmq",
 	"rdp",
 	"redis",
+	"imap",
 	"jdwp",
+	"pop3",
 	"rsync",
 	"smb",
 	"smtp",
@@ -537,6 +539,7 @@ var serviceAuthPlugins = map[string]bool{
 	"cassandra":     true,
 	"elasticsearch": true,
 	"ftp":           true,
+	"imap":          true,
 	"kafka":         true,
 	"ldap":          true,
 	"memcached":     true,
@@ -545,6 +548,7 @@ var serviceAuthPlugins = map[string]bool{
 	"mysql":         true,
 	"neo4j":         true,
 	"oracle":        true,
+	"pop3":          true,
 	"postgresql":    true,
 	"rabbitmq":      true,
 	"redis":         true,