Add VideoWall feature: server-side preview clip generation and mobile grid view

Backend (Rust/Actix-web):
- Add video_preview_clips table and PreviewDao for tracking preview generation
- Add ffmpeg preview clip generator: 10 equally-spaced 1s segments at 480p with CUDA NVENC auto-detection
- Add PreviewClipGenerator actor with semaphore-limited concurrent processing
- Add GET /video/preview and POST /video/preview/status endpoints
- Extend file watcher to detect and queue previews for new videos
- Use relative paths consistently for DB storage (matching EXIF convention)

Frontend (React Native/Expo):
- Add VideoWall grid view with 2-3 column layout of looping preview clips
- Add VideoWallItem component with ActiveVideoPlayer sub-component for lifecycle management
- Add useVideoWall hook for batch status polling with 5s refresh
- Add navigation button in grid header (visible when videos exist)
- Use TextureView surface type to fix Android z-ordering issues
- Optimize memory: players only mount while visible via FlatList windowSize
- Configure ExoPlayer buffer options and caching for short clips
- Tap to toggle audio focus, long press to open in full viewer

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

specs/001-video-wall/quickstart.md

@@ -0,0 +1,115 @@
+# Quickstart: VideoWall
+
+## Prerequisites
+
+- Rust toolchain (stable) with `cargo`
+- `diesel_cli` installed (`cargo install diesel_cli --no-default-features --features sqlite`)
+- ffmpeg and ffprobe available on PATH
+- Node.js 18+ and Expo CLI for mobile app
+- `.env` file configured with existing variables plus `PREVIEW_CLIPS_DIRECTORY`
+
+## New Environment Variable
+
+Add to `.env`:
+```bash
+PREVIEW_CLIPS_DIRECTORY=/path/to/preview-clips   # Directory for generated preview MP4s
+```
+
+## Backend Development
+
+### 1. Create database migration
+
+```bash
+cd C:\Users\ccord\RustroverProjects\ImageApi
+diesel migration generate create_video_preview_clips
+```
+
+Edit the generated `up.sql`:
+```sql
+CREATE TABLE video_preview_clips (
+    id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
+    file_path TEXT NOT NULL UNIQUE,
+    status TEXT NOT NULL DEFAULT 'pending',
+    duration_seconds REAL,
+    file_size_bytes INTEGER,
+    error_message TEXT,
+    created_at TEXT NOT NULL,
+    updated_at TEXT NOT NULL
+);
+
+CREATE INDEX idx_preview_clips_file_path ON video_preview_clips(file_path);
+CREATE INDEX idx_preview_clips_status ON video_preview_clips(status);
+```
+
+Edit `down.sql`:
+```sql
+DROP TABLE IF EXISTS video_preview_clips;
+```
+
+Regenerate schema:
+```bash
+diesel migration run
+diesel print-schema > src/database/schema.rs
+```
+
+### 2. Build and test backend
+
+```bash
+cargo build
+cargo test
+cargo run
+```
+
+Test preview endpoint:
+```bash
+# Check preview status
+curl -X POST http://localhost:8080/video/preview/status \
+  -H "Authorization: Bearer <token>" \
+  -H "Content-Type: application/json" \
+  -d '{"paths": ["some/video.mp4"]}'
+
+# Request preview clip
+curl http://localhost:8080/video/preview?path=some/video.mp4 \
+  -H "Authorization: Bearer <token>" \
+  -o preview.mp4
+```
+
+### 3. Verify preview clip generation
+
+Check that preview clips appear in `PREVIEW_CLIPS_DIRECTORY` with the expected directory structure mirroring `BASE_PATH`.
+
+## Frontend Development
+
+### 1. Start the mobile app
+
+```bash
+cd C:\Users\ccord\development\SynologyFileViewer
+npx expo start
+```
+
+### 2. Navigate to VideoWall
+
+From the grid view of any folder containing videos, switch to VideoWall mode. The view should display a 2-3 column grid of looping preview clips.
+
+## Key Files to Modify
+
+### Backend (ImageApi)
+| File | Change |
+|------|--------|
+| `src/video/ffmpeg.rs` | Add `generate_preview_clip()` function |
+| `src/video/actors.rs` | Add `PreviewClipGenerator` actor |
+| `src/video/mod.rs` | Add `generate_preview_clips()` batch function |
+| `src/main.rs` | Add endpoints, extend file watcher |
+| `src/database/schema.rs` | Regenerated by Diesel |
+| `src/database/models.rs` | Add `VideoPreviewClip` struct |
+| `src/database/preview_dao.rs` | New DAO file |
+| `src/data/mod.rs` | Add request/response types |
+| `src/state.rs` | Add PreviewClipGenerator to AppState |
+
+### Frontend (SynologyFileViewer)
+| File | Change |
+|------|--------|
+| `app/(app)/grid/video-wall.tsx` | New VideoWall view |
+| `app/(app)/grid/_layout.tsx` | Add route |
+| `components/VideoWallItem.tsx` | New preview clip cell component |
+| `hooks/useVideoWall.ts` | New hook for preview state management |