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/plan.md

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+# Implementation Plan: VideoWall
+
+**Branch**: `001-video-wall` | **Date**: 2026-02-25 | **Spec**: [spec.md](./spec.md)
+**Input**: Feature specification from `/specs/001-video-wall/spec.md`
+
+## Summary
+
+Add a VideoWall feature spanning the Rust API backend and React Native mobile app. The backend generates 480p MP4 preview clips (up to 10 seconds, composed of 10 equally spaced 1-second segments) using ffmpeg, extending the existing `OverviewVideo` pattern in `src/video/ffmpeg.rs`. The mobile app adds a VideoWall view using `expo-video` and FlatList to display a responsive 2-3 column grid of simultaneously looping, muted preview clips with audio-on-long-press. Preview clips are cached on disk, served via new API endpoints, and generated proactively by the file watcher.
+
+## Technical Context
+
+**Language/Version**: Rust (stable, Cargo) for backend API; TypeScript / React Native (Expo SDK 52) for mobile app
+**Primary Dependencies**: actix-web 4, Diesel 2.2 (SQLite), ffmpeg/ffprobe (CLI), expo-video 3.0, expo-router 6.0, react-native-reanimated 4.1
+**Storage**: SQLite (preview clip status tracking), filesystem (MP4 preview clips in `PREVIEW_CLIPS_DIRECTORY`)
+**Testing**: `cargo test` for backend; manual testing for mobile app
+**Target Platform**: Linux server (API), iOS/Android (mobile app via Expo)
+**Project Type**: Mobile app + REST API (two separate repositories)
+**Performance Goals**: <3s VideoWall load for 50 pre-generated previews; <30s per clip generation; <5MB per clip; smooth simultaneous playback of 6-12 clips
+**Constraints**: Semaphore-limited concurrent ffmpeg processes (existing pattern); 480p resolution to keep bandwidth/CPU manageable; audio track preserved but muted by default
+**Scale/Scope**: Hundreds to low thousands of videos per library; single user at a time
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+Constitution is an unfilled template — no project-specific gates defined. **PASS** (no violations possible).
+
+Post-Phase 1 re-check: Still PASS — no gates to evaluate.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/001-video-wall/
+├── plan.md              # This file
+├── research.md          # Phase 0 output
+├── data-model.md        # Phase 1 output
+├── quickstart.md        # Phase 1 output
+├── contracts/           # Phase 1 output
+│   └── api-endpoints.md
+└── tasks.md             # Phase 2 output (/speckit.tasks command)
+```
+
+### Source Code (repository root)
+
+```text
+# Backend (ImageApi - Rust)
+src/
+├── video/
+│   ├── ffmpeg.rs            # Add generate_preview_clip() using existing pattern
+│   ├── actors.rs            # Add PreviewClipGenerator actor (semaphore-limited)
+│   └── mod.rs               # Add generate_preview_clips() batch function
+├── main.rs                  # Add GET /video/preview, POST /video/preview/status endpoints
+│                            # Extend file watcher to trigger preview generation
+├── database/
+│   ├── schema.rs            # Add video_preview_clips table
+│   └── models.rs            # Add VideoPreviewClip model
+│   └── preview_dao.rs       # New DAO for preview clip status tracking
+└── data/
+    └── mod.rs               # Add PreviewClipRequest, PreviewStatusRequest types
+
+# Frontend (SynologyFileViewer - React Native)
+app/(app)/grid/
+├── video-wall.tsx           # New VideoWall view (FlatList grid)
+└── _layout.tsx              # Add video-wall route to stack
+
+components/
+└── VideoWallItem.tsx        # Single preview clip cell (expo-video player)
+
+hooks/
+└── useVideoWall.ts          # Preview clip fetching, status polling, audio state
+```
+
+**Structure Decision**: Mobile + API pattern. Backend changes extend existing `src/video/` module and `src/main.rs` handlers following established conventions. Frontend adds a new route under the existing grid stack navigator with a dedicated component and hook.
+
+## Complexity Tracking
+
+No constitution violations to justify.