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/config.py

@@ -22,6 +22,7 @@ class FilenameConfig:
 
 @dataclass
 class AppConfig:
+    storage: str  # required: 'local' or 's3'
     display_max: int = 480
     fps_fallback: int = 25
     max_frames: int = 100
@@ -71,6 +72,10 @@ def load_optical_flow_config(path: Path) -> OpticalFlowConfig:
 def load_config(path: Path) -> AppConfig:
     with open(path) as f:
         data = yaml.safe_load(f)
+    if "storage" not in data:
+        raise ValueError(
+            f"{path}: missing required field 'storage'. Set it to 'local' or 's3'."
+        )
     fn_data = data.pop("filenames", {})
     cfg = AppConfig(**data)
     cfg.filenames = FilenameConfig(**fn_data)