Add S3 storage support via s3fs; make storage field required

- New filesystem.py: make_fs() factory (returns s3fs.S3FileSystem or None),
  plus fsjoin/fsstem/fsname path helpers
- config.py: storage field is now required ('local' or 's3'); load_config
  raises a clear ValueError when it is missing
- video_loader, clip_selector, annotator: thread fs through all file I/O;
  local paths unchanged, S3 paths use fs.open/fs.exists/fs.pipe
- annotation_script: load .env via python-dotenv at startup, create fs from
  config and pass to Annotator
- Add .env.example with SwitchEngines endpoint and AWS checksum env vars
- pyproject.toml: add s3fs and python-dotenv dependencies
- Reduce default mask alpha from 40% to 15%
- Update example clip names to colon-separated timestamps

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

src/river_annotation_tool/mask_canvas.py

@@ -11,7 +11,7 @@ class MaskCanvas:
     """Matplotlib canvas with brush/polygon mask drawing, undo/redo, and erase."""
 
     _BRUSH_DEFAULT = 5
-    _ALPHA_DEFAULT = 40
+    _ALPHA_DEFAULT = 15
     _BRIGHTNESS_DEFAULT = 0
     _CONTRAST_DEFAULT = 0
     _GAMMA_DEFAULT = 100