Add FTP/SFTP support and tree-view folder picker with lazy loading

FTP/SFTP processor:
- New ftp_processor.py with adapter pattern for FTP (passive) and SFTP
- Same design as smb_processor: read PDFs, forward via SMTP, move to processed
- Eingangs-/Ausgangsbelege with separate paths, modes (forward/separator)
- paramiko==3.5.0 for SFTP support
- Schema v9 with new ftp_* settings
- Integrated in scheduler

Tree-view folder picker (SMB + FTP):
- Reusable tree rendering from flat path lists
- Expandable/collapsible nodes with toggle arrows
- Lazy loading: only top-level folders on open, sub-folders on-demand
- Auto-expand ancestors of currently selected value (with preload)
- Reload button stays for manual refresh
- Always fresh load when opening picker
- New endpoints: /api/list-smb-subfolders, /api/list-ftp-subfolders

FTP-specific fixes:
- list_pdfs uses LIST instead of NLST (more reliable across servers)
- Stateful CWD bug fixed in ensure_dir/stat_exists/rename
  (previously created /Buch/Buch/X instead of /Buch/X due to CWD drift)
- All operations reset CWD via _reset_cwd() before stateful calls
- _resolve() helper for SFTP to handle empty path / chroot users

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/database.py

@@ -4,13 +4,13 @@ import aiosqlite
 from cryptography.fernet import Fernet
 
 DB_PATH = os.environ.get("DB_PATH", "/data/belegimport.db")
-SCHEMA_VERSION = 8
+SCHEMA_VERSION = 9
 
 logger = logging.getLogger(__name__)
 
 _fernet = None
 
-ENCRYPTED_KEYS = {"imap_password", "smtp_password", "smb_password", "amazon_password", "amazon_client_secret", "amazon_refresh_token"}
+ENCRYPTED_KEYS = {"imap_password", "smtp_password", "smb_password", "amazon_password", "amazon_client_secret", "amazon_refresh_token", "ftp_password"}
 
 DEFAULT_SETTINGS = {
     "imap_server": "",
@@ -46,6 +46,18 @@ DEFAULT_SETTINGS = {
     "smb_source_path_ausgang": "",
     "smb_processed_path_ausgang": "",
     "smb_mode": "forward",
+    # FTP / SFTP
+    "ftp_enabled": "false",
+    "ftp_protocol": "sftp",  # "sftp" or "ftp"
+    "ftp_server": "",
+    "ftp_port": "22",
+    "ftp_username": "",
+    "ftp_password": "",
+    "ftp_source_path": "",
+    "ftp_processed_path": "Verarbeitet",
+    "ftp_source_path_ausgang": "",
+    "ftp_processed_path_ausgang": "",
+    "ftp_mode": "forward",
     # Amazon
     "amazon_enabled": "false",
     "amazon_email": "",
@@ -241,6 +253,12 @@ async def _run_migrations(db: aiosqlite.Connection, current_version: int):
         await db.commit()
         await _set_schema_version(db, 8)
 
+    if current_version < 9:
+        logger.info("Migration v9: FTP/SFTP-Settings hinzugefuegt (defaults werden eingefuegt)")
+        # No table changes needed - new settings are added via DEFAULT_SETTINGS loop in init_db
+        await db.commit()
+        await _set_schema_version(db, 9)
+
     await db.commit()