fix: Create S3 folder markers after bisync for TigrisFS visibility

TigrisFS requires folder marker objects (empty S3 objects with keys ending
in '/') to recognize directory structure. Rclone bisync does not create
these markers automatically, causing synced files to be invisible in the
TigrisFS FUSE mount.

Changes:
- Add boto3 dependency for S3 operations
- Implement get_s3_credentials_from_rclone() to extract credentials from rclone config
- Implement create_folder_markers() to scan local directories and create missing markers
- Integrate folder marker creation after successful bisync operations
- Add proper error handling that warns but doesn't fail bisync

Fixes #460

Co-authored-by: Drew Cain <groksrc@users.noreply.github.com>

pyproject.toml

@@ -38,6 +38,7 @@ dependencies = [
     "nest-asyncio>=1.6.0", # For Alembic migrations with Postgres
     "pytest-asyncio>=1.2.0",
     "psycopg==3.3.1",
+    "boto3>=1.35.0", # For S3 folder marker creation in cloud sync
 ]