feat: implement export engine

Add async ZIP and HTML offline viewer export workers, download endpoints,
and a guest-facing /export page.

Backend — export workers (tokio::spawn, run after gallery release):
- ZIP worker: streams all non-deleted originals into Gallery.zip via
  async_zip (Stored compression), organised into Photos/ and Videos/
  with {date}_{uploader}_{id}.{ext} filenames; updates progress_pct in DB
- HTML worker: renders Memories.html via minijinja template (self-contained:
  inlined CSS + JS, relative media paths); packs it with README.txt and
  all media into Memories.zip (Deflate for text, Stored for media)
- Both workers mark export_job status (running → done/failed), update
  export_zip_ready / export_html_ready on the event, and broadcast SSE
  export-progress + export-available when both complete

Backend — new endpoints (AuthUser):
- GET /export/zip   → streams Gallery.zip if export_zip_ready
- GET /export/html  → streams Memories.zip if export_html_ready
- GET /export/status → released flag + per-type status/progress (moved from admin)

Memories.html features: warm keepsake aesthetic, responsive grid, fullscreen
lightbox with captions/comments/likes, client-side hashtag filter chips,
XSS-safe JS, fully offline (no external deps)

Frontend — /export page:
- Locked state: padlock illustration + message
- Released state: ZIP and HTML cards with progress bars (SSE-driven),
  download buttons enabled only when ready
- HTML guide modal (unzip instructions + Wi-Fi tip) before download begins

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Cargo.lock

@@ -907,6 +907,7 @@ dependencies = [
  "sysinfo",
  "tokio",
  "tokio-stream",
+ "tokio-util",
  "tower",
  "tower-http",
  "tower_governor",