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/downloads/og.py

@@ -0,0 +1,85 @@
+"""Server-rendered Open Graph tags for the public /watch/{token} page.
+
+The watch page is a client-side SPA, so a social crawler (facebookexternalhit, Twitterbot, …) that
+fetches /watch/{token} would otherwise only see the generic index.html — a blank/greyed link card.
+This injects per-video OG/Twitter tags into the served HTML so a shared link unfurls with the
+video's title, channel and thumbnail. Real browsers ignore the extra tags and hydrate the SPA as
+usual. Password-protected / expired / invalid links fall back to the generic card (no metadata leak).
+"""
+import html
+import re
+from pathlib import Path
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.config import settings
+from app.downloads import links as linksmod
+from app.models import DownloadJob, DownloadLink, MediaAsset
+
+_OG_BLOCK = re.compile(r"<!--OG:START-->.*?<!--OG:END-->", re.DOTALL)
+
+
+def _tag(prop: str, content: str, attr: str = "property") -> str:
+    return f'<meta {attr}="{prop}" content="{html.escape(content, quote=True)}" />'
+
+
+def _watch_tags(token: str, db: Session) -> str | None:
+    """Build the OG/Twitter meta block for a watch token, or None to keep the generic card."""
+    link = db.execute(
+        select(DownloadLink).where(DownloadLink.token == token)
+    ).scalar_one_or_none()
+    if link is None or linksmod.is_expired(link):
+        return None
+    url = f"{settings.app_base}/watch/{token}"
+    if link.password_hash:
+        # Password-gated: don't leak the title/thumbnail in the unfurl.
+        return "\n    ".join(
+            [
+                "<title>Siftlode</title>",
+                _tag("og:title", "Password-protected video"),
+                _tag("og:description", "Shared via Siftlode"),
+                _tag("og:type", "video.other"),
+                _tag("og:url", url),
+                _tag("og:site_name", "Siftlode"),
+            ]
+        )
+    job = db.get(DownloadJob, link.job_id)
+    asset = db.get(MediaAsset, job.asset_id) if job and job.asset_id else None
+    if job is None or asset is None or asset.status != "ready":
+        return None
+    title = (job.display_name or asset.title or "Video").strip()
+    channel = (job.display_uploader or asset.uploader or "").strip()
+    image = asset.thumbnail_url  # already an absolute (youtube/fb) URL, or None
+    tags = [
+        f"<title>{html.escape(title)}</title>",
+        _tag("og:title", title),
+        _tag("og:description", channel or "Shared via Siftlode"),
+        _tag("og:type", "video.other"),
+        _tag("og:url", url),
+        _tag("og:site_name", "Siftlode"),
+        _tag("twitter:title", title, attr="name"),
+    ]
+    if channel:
+        tags.append(_tag("twitter:description", channel, attr="name"))
+    if image:
+        # A real thumbnail → a large image card; without one, a plain (text) card is served.
+        tags.append(_tag("og:image", image))
+        tags.append(_tag("twitter:card", "summary_large_image", attr="name"))
+        tags.append(_tag("twitter:image", image, attr="name"))
+    else:
+        tags.append(_tag("twitter:card", "summary", attr="name"))
+    return "\n    ".join(tags)
+
+
+def render_watch_html(token: str, index_html: Path, db: Session) -> str | None:
+    """Return index.html with the OG block replaced for this watch token, or None to serve the
+    generic page unchanged (invalid/expired/not-ready link, or a template without the markers)."""
+    tags = _watch_tags(token, db)
+    if tags is None:
+        return None
+    text = index_html.read_text(encoding="utf-8")
+    if "<!--OG:START-->" not in text:
+        return None
+    # A function replacement avoids re.sub interpreting backslashes/group refs in `tags`.
+    return _OG_BLOCK.sub(lambda _m: tags, text, count=1)