feat(v5.5.0): Embeds & Media riche - universal embeds, bookmark cards, lightbox, inline previews, cover & icon

- embeds.py: provider detection/rewrite (YouTube, Vimeo, Figma, Maps, Docs,
  Loom, CodePen, Miro, Spotify, SoundCloud, Twitch, X/Twitter, Pinterest,
  Office) + resolve_embed/inline_kind/provider; POST /board/api/embed/resolve
- editor resolves pasted URLs and caches embed_src (persisted); renderer,
  public pages and MD/HTML/PDF exports prefer embed_src
- og_fetcher.py: robust meta parsing (any attribute order), favicon,
  injectable transport, network-safe fallback
- image lightbox with keyboard nav (arrows/Esc) in editor and public pages
- inline PDF/video/audio previews
- cover (URL or upload) & page icon endpoints
- fix broken editor API paths (/api/pages -> /board/api/pages) for cover,
  icon, versions, backlinks, import, move and OG metadata
- 47 tests in tests/test_v55.py; full suite 444 green; ruff clean
- version 5.11.2

app/services/export.py

@@ -665,7 +665,7 @@ def blocks_to_html(blocks: list) -> str:
                 parts.append(f'<iframe src="{_text(url)}" style="width:100%;height:70vh;border:none;border-radius:8px;"></iframe>')
             elif url:
                 from app.services.embeds import embed_src
-                src = embed_src(url) or url
+                src = b.get("embed_src") or embed_src(url) or url
                 height = 520
                 if b.get("height"):
                     try: