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>

migrations/2026-02-25-221921-0000_create_video_preview_clips/up.sql

@@ -0,0 +1,13 @@
+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);