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/contracts/api-endpoints.md

@@ -0,0 +1,91 @@
+# API Contracts: VideoWall
+
+## GET /video/preview
+
+Retrieve the preview clip MP4 file for a given video. If the preview is not yet generated, triggers on-demand generation and returns 202.
+
+**Authentication**: Required (Bearer token)
+
+**Query Parameters**:
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| path | string | yes | Relative path of the source video from BASE_PATH |
+
+**Responses**:
+
+| Status | Content-Type | Body | Description |
+|--------|-------------|------|-------------|
+| 200 | video/mp4 | MP4 file stream | Preview clip is ready and served |
+| 202 | application/json | `{"status": "processing", "path": "<path>"}` | Preview generation has been triggered; client should retry |
+| 400 | application/json | `{"error": "Invalid path"}` | Path validation failed |
+| 404 | application/json | `{"error": "Video not found"}` | Source video does not exist |
+| 500 | application/json | `{"error": "Generation failed: <detail>"}` | Preview generation failed |
+
+**Behavior**:
+1. Validate path with `is_valid_full_path()`
+2. Check if preview clip exists on disk and status is `complete` → serve MP4 (200)
+3. If status is `pending` or no record exists → trigger generation, return 202
+4. If status is `processing` → return 202
+5. If status is `failed` → return 500 with error detail
+
+---
+
+## POST /video/preview/status
+
+Check the preview generation status for a batch of video paths. Used by the mobile app to determine which previews are ready before requesting them.
+
+**Authentication**: Required (Bearer token)
+
+**Request Body** (application/json):
+
+```json
+{
+  "paths": [
+    "2024/vacation/beach.mov",
+    "2024/vacation/sunset.mp4",
+    "2024/birthday.avi"
+  ]
+}
+```
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| paths | string[] | yes | Array of relative video paths from BASE_PATH |
+
+**Response** (200, application/json):
+
+```json
+{
+  "previews": [
+    {
+      "path": "2024/vacation/beach.mov",
+      "status": "complete",
+      "preview_url": "/video/preview?path=2024/vacation/beach.mov"
+    },
+    {
+      "path": "2024/vacation/sunset.mp4",
+      "status": "processing",
+      "preview_url": null
+    },
+    {
+      "path": "2024/birthday.avi",
+      "status": "pending",
+      "preview_url": null
+    }
+  ]
+}
+```
+
+| Field | Type | Description |
+|-------|------|-------------|
+| previews | object[] | Status for each requested path |
+| previews[].path | string | The requested video path |
+| previews[].status | string | One of: `pending`, `processing`, `complete`, `failed`, `not_found` |
+| previews[].preview_url | string? | Relative URL to fetch the preview (only when status is `complete`) |
+
+**Behavior**:
+1. Accept up to 200 paths per request
+2. Batch query the `video_preview_clips` table for all paths
+3. For paths not in the table, return status `not_found` (video may not exist or hasn't been scanned yet)
+4. Return results in the same order as the input paths