feat: add VitePress docs site, music file picker modal, Dockerfile optimization, i18n keys for 14 locales

- Add docs/ with VitePress site (19 pages): guide, config, architecture, FAQ
- Add GitHub Actions workflow for auto-deploy to GitHub Pages
- Replace music 'Add Tracks' upload picker with in-app audio file browser modal
- Add music picker CSS styles with dark theme support
- Add missing i18n keys (search_audio, no_audio_files, etc.) to all 14 locales
- Optimize Dockerfile: shared base stage, COPY --chmod, consolidated RUN, HEALTHCHECK
- Improve README: docs links, updated stats (222+ tests, 14 languages), feature status

docs/architecture/caching.md

@@ -0,0 +1,37 @@
+# Caching Architecture
+
+OxiCloud uses **moka** (a lock-free, concurrent cache) for write-behind caching that delivers sub-millisecond hot reads.
+
+## Cache Layers
+
+| Cache | TTL | Max Entries | Purpose |
+|---|---|---|---|
+| File metadata | 60 s | 10 000 | Avoid re-querying PostgreSQL for file info |
+| Directory listings | 120 s | 10 000 | Frequently accessed folder contents |
+| Thumbnail cache | configurable | 1 000 | Generated WebP/AVIF thumbnails |
+| Image transcode | configurable | 500 | On-the-fly image transcoding results |
+| Blob hash | 30 s TTI | 5 000 | SHA-256 hashes for dedup lookups |
+| Audio metadata | — | 2 000 | ID3 tags and duration |
+
+## How It Works
+
+1. **Read path:** check cache → if hit, return immediately (sub-ms); if miss, query PostgreSQL, populate cache, return
+2. **Write path:** update PostgreSQL → invalidate relevant cache entries
+3. **TTL expiry:** entries are evicted after their time-to-live, ensuring eventual consistency
+
+## Why moka?
+
+- **Lock-free** — no mutex contention under concurrent access
+- **Bounded memory** — max entries prevent unbounded growth
+- **TTL + TTI** — supports both time-to-live and time-to-idle eviction
+- **Async-ready** — works natively with Tokio
+
+## Configuration
+
+Cache parameters are currently hardcoded in `src/common/config.rs`. Key defaults:
+
+```rust
+file_cache_ttl_ms: 60_000,       // 1 minute
+directory_cache_ttl_ms: 120_000,  // 2 minutes
+max_cache_entries: 10_000,
+```