feat(share): rich link previews (Open Graph) for /watch pages

A shared /watch/{token} link is a client-rendered SPA, so a social crawler only
saw the generic index.html — a blank link card. The server now injects per-video
Open Graph / Twitter tags (title, channel, thumbnail) into the served HTML for
that route, so links unfurl richly in Messenger and other chat apps; real
browsers ignore the extra tags and hydrate the page as usual. Password /
expired / invalid links fall back to the generic card with no metadata leak.

Also shortens the generic site description used for search engines and link
previews.

backend/app/main.py

@@ -20,7 +20,7 @@ if not _siftlode_logger.handlers:
 
 log = logging.getLogger("siftlode.app")
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import FileResponse, JSONResponse
+from fastapi.responses import FileResponse, HTMLResponse, JSONResponse
 from fastapi.staticfiles import StaticFiles
 from starlette.middleware.sessions import SessionMiddleware
 
@@ -178,6 +178,21 @@ async def index() -> FileResponse:
     return FileResponse(INDEX_HTML, headers=INDEX_HEADERS)
 
 
+@app.get("/watch/{token}")
+async def watch_page(token: str):
+    """Serve the SPA for a public share link, but with per-video Open Graph tags injected so the
+    link unfurls richly (title/channel/thumbnail) in Messenger/social crawlers. A real browser
+    ignores the extra tags and hydrates WatchPage as usual; invalid/expired/password links fall
+    back to the generic card."""
+    from app.downloads import og
+
+    with SessionLocal() as db:
+        rendered = og.render_watch_html(token, INDEX_HTML, db)
+    if rendered is None:
+        return FileResponse(INDEX_HTML, headers=INDEX_HEADERS)
+    return HTMLResponse(rendered, headers=INDEX_HEADERS)
+
+
 @app.get("/{full_path:path}")
 async def spa_fallback(full_path: str) -> FileResponse:
     # Client-side routes fall back to index.html; real API/asset paths are matched above.