feat: replace SFTP drop with Syncthing share path

Remove SFTPGo; mount event data from the Syncthing folder, watch
incoming/, and name variants after the source stem.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -1,7 +1,7 @@
 # Secrets
 .env
 
-# Runtime data (bind-mounted, never belongs in git)
+# Runtime data (bind-mounted / Syncthing share — never in git)
 /data/
 
 # Local rembg/withoutbg comparison scratch
@@ -14,8 +14,15 @@
 # Python
 __pycache__/
 *.pyc
+*.pyo
 .venv/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
 
 # OS / editor noise
 .DS_Store
 *.swp
+*~
+.idea/
+.vscode/