feat(PROJ-26): IMAP-Archive-Server Read-Only Zugriff auf archivierte Mails

- Neues Package internal/imapserver: vollständiger IMAP4rev1-Server (~700 Zeilen)
- Auth via bcrypt (userstore.VerifyPassword), Multi-Tenant-Isolation
- INBOX + INBOX/LabelName Ordnerstruktur
- FETCH mit BODY[], ENVELOPE, RFC822.SIZE, INTERNALDATE, FLAGS, UID
- SEARCH: ALL, FROM, TO, SUBJECT, SINCE, BEFORE + UID FETCH/SEARCH
- Read-Only: STORE, DELETE, COPY, MOVE, APPEND → NO [CANNOT]
- \Seen-Flag nicht persistent (GoBD-konform)
- Max 5 gleichzeitige Verbindungen pro User, 30min Idle-Timeout
- Audit-Log: imap_login / imap_login_failed Events
- Config: imap_server.enabled + imap_server.bind (default: 127.0.0.1:1143)
- Externe Ports: 9993 (primär) und 993 (alternativ) via nginx TLS-Terminierung

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/config.go

@@ -21,14 +21,21 @@ type APIConfig struct {
 
 // Config is the full application configuration loaded from YAML.
 type Config struct {
-	Server   ServerConfig   `yaml:"server"`
-	Storage  StorageConfig  `yaml:"storage"`
-	Database DatabaseConfig `yaml:"database"`
-	SMTP     SMTPConfig     `yaml:"smtp"`
-	API      APIConfig      `yaml:"api"`
-	Index    IndexConfig    `yaml:"index"`
-	Audit    AuditConfig    `yaml:"audit"`
-	Logging  LoggingConfig  `yaml:"logging"`
+	Server     ServerConfig     `yaml:"server"`
+	Storage    StorageConfig    `yaml:"storage"`
+	Database   DatabaseConfig   `yaml:"database"`
+	SMTP       SMTPConfig       `yaml:"smtp"`
+	API        APIConfig        `yaml:"api"`
+	Index      IndexConfig      `yaml:"index"`
+	Audit      AuditConfig      `yaml:"audit"`
+	Logging    LoggingConfig    `yaml:"logging"`
+	IMAPServer IMAPServerConfig `yaml:"imap_server"`
+}
+
+// IMAPServerConfig holds settings for the embedded read-only IMAP archive server.
+type IMAPServerConfig struct {
+	Enabled bool   `yaml:"enabled"`
+	Bind    string `yaml:"bind"` // default: "127.0.0.1:1143"
 }
 
 // ServerConfig holds port settings for the main services.