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>

src/database/models.rs

@@ -1,4 +1,4 @@
-use crate::database::schema::{favorites, image_exif, photo_insights, users};
+use crate::database::schema::{favorites, image_exif, photo_insights, users, video_preview_clips};
 use serde::Serialize;
 
 #[derive(Insertable)]
@@ -93,3 +93,24 @@ pub struct PhotoInsight {
     pub generated_at: i64,
     pub model_version: String,
 }
+
+#[derive(Insertable)]
+#[diesel(table_name = video_preview_clips)]
+pub struct InsertVideoPreviewClip {
+    pub file_path: String,
+    pub status: String,
+    pub created_at: String,
+    pub updated_at: String,
+}
+
+#[derive(Serialize, Queryable, Clone, Debug)]
+pub struct VideoPreviewClip {
+    pub id: i32,
+    pub file_path: String,
+    pub status: String,
+    pub duration_seconds: Option<f32>,
+    pub file_size_bytes: Option<i32>,
+    pub error_message: Option<String>,
+    pub created_at: String,
+    pub updated_at: String,
+}