Add comprehensive testing for preview clip and status handling

- Implement unit tests for PreviewClipRequest/PreviewStatusRequest serialization and deserialization.
- Add tests for PreviewDao (insert, update, batch retrieval, and status-based queries).
- Extend Actix-web integration tests for `/video/preview/status` endpoint scenarios.
- Introduce in-memory TestPreviewDao for mock database interactions.
- Update README with new config parameters for preview clips.

README.md

@@ -9,6 +9,7 @@ Upon first run it will generate thumbnails for all images and videos at `BASE_PA
 - Video streaming with HLS
 - Tag-based organization
 - Memories API for browsing photos by date
+- **Video Wall** - Auto-generated short preview clips for videos, served via a grid view
 - **AI-Powered Photo Insights** - Generate contextual insights from photos using LLMs
 - **RAG-based Context Retrieval** - Semantic search over daily conversation summaries
 - **Automatic Daily Summaries** - LLM-generated summaries of daily conversations with embeddings
@@ -22,10 +23,12 @@ You must have `ffmpeg` installed for streaming video and generating video thumbn
 - `BASE_PATH` is the root from which you want to serve images and videos
 - `THUMBNAILS` is a path where generated thumbnails should be stored
 - `VIDEO_PATH` is a path where HLS playlists and video parts should be stored
+- `GIFS_DIRECTORY` is a path where generated video GIF thumbnails should be stored
 - `BIND_URL` is the url and port to bind to (typically your own IP address)
 - `SECRET_KEY` is the *hopefully* random string to sign Tokens with
 - `RUST_LOG` is one of `off, error, warn, info, debug, trace`, from least to most noisy [error is default]
 - `EXCLUDED_DIRS` is a comma separated list of directories to exclude from the Memories API
+- `PREVIEW_CLIPS_DIRECTORY` (optional) is a path where generated video preview clips should be stored [default: `preview_clips`]
 - `WATCH_QUICK_INTERVAL_SECONDS` (optional) is the interval in seconds for quick file scans [default: 60]
 - `WATCH_FULL_INTERVAL_SECONDS` (optional) is the interval in seconds for full file scans [default: 3600]