fix(share): use the stable YouTube hqdefault thumbnail for OG on YT shares (0.57.1)

Facebook's scraper rejected our self-hosted poster as "Corrupted Image": the poster is an
ffmpeg-produced JPEG (non-standard marker order) that browsers/iOS/Twitter decode fine but FB's
stricter processor won't. For a YouTube source, point og:image at the video's stable, non-expiring
`i.ytimg.com/vi/<id>/hqdefault.jpg` (a Google-encoded JPEG FB decodes reliably) built from the id in
the stored thumbnail — instead of the stored `sqp`-signed variant, which can expire. Non-YouTube
sources keep the poster fallback (fine everywhere but FB). Declares 480×360 so FB includes it on the
first async scrape. og.py only — no schema change.

backend/app/downloads/og.py

@@ -95,15 +95,22 @@ def _watch_tags(token: str, db: Session) -> str | None:
         return None
     title = (job.display_name or asset.title or "Video").strip()
     channel = (job.display_uploader or asset.uploader or "").strip()
-    # Prefer our self-hosted poster for the link-preview image: a remote thumbnail (Facebook's
-    # signed CDN URL especially) expires and isn't reliably fetchable cross-origin by the crawler.
-    # (This block only runs for non-password links, so the poster needs no grant.) Fall back to the
-    # remote thumbnail only if we somehow have no poster.
-    image = (
-        f"{settings.app_base}/api/public/watch/{token}/poster.jpg"
-        if asset.poster_path
-        else asset.thumbnail_url
-    )
+    # Pick the link-preview image. Facebook's scraper REJECTS our own poster ("Corrupted Image" in the
+    # Sharing Debugger): the poster is an ffmpeg-produced JPEG whose non-standard marker order FB's
+    # decoder won't accept, even though browsers/iOS/Twitter render it fine. So for a YouTube source
+    # prefer its STABLE hqdefault thumbnail — a Google-encoded JPEG (480×360) at a non-expiring URL
+    # (unlike the stored `sqp`-signed variant) that FB decodes reliably. Non-YouTube sources fall back
+    # to the poster (fine everywhere but FB) then the raw remote thumbnail. This block only runs for
+    # non-password links, so the poster needs no grant.
+    yt_id_match = re.search(r"i\.ytimg\.com/vi/([\w-]+)/", asset.thumbnail_url or "")
+    image_dims: tuple[int, int] | None = None
+    if yt_id_match:
+        image: str | None = f"https://i.ytimg.com/vi/{yt_id_match.group(1)}/hqdefault.jpg"
+        image_dims = (480, 360)
+    elif asset.poster_path:
+        image = f"{settings.app_base}/api/public/watch/{token}/poster.jpg"
+    else:
+        image = asset.thumbnail_url
     tags = [
         f"<title>{html.escape(title)}</title>",
         _tag("og:title", title),
@@ -118,20 +125,20 @@ def _watch_tags(token: str, db: Session) -> str | None:
     if image:
         # A real thumbnail → a large image card; without one, a plain (text) card is served.
         tags.append(_tag("og:image", image))
-        # Facebook's scraper is pickier than Apple's on-device unfurl: spell out that the image is a
-        # public HTTPS JPEG, and declare its dimensions so the card includes the image on the very
-        # first (async) scrape instead of unfurling title-only — see _jpeg_size.
+        # Declare the image is a public HTTPS JPEG with known dimensions, so FB (which fetches
+        # og:image asynchronously on first scrape) includes it instead of unfurling title-only. For a
+        # poster we read the real dimensions from its JPEG header; the ytimg hqdefault is a fixed 480×360.
         if image.startswith("https://"):
             tags.append(_tag("og:image:secure_url", image))
-        if image.endswith("poster.jpg") and asset.poster_path:
             tags.append(_tag("og:image:type", "image/jpeg"))
+        if image_dims is None and image.endswith("poster.jpg") and asset.poster_path:
             # safe_abs_path returns None if the poster file is missing (gc'd) or escapes the root —
             # guard it, else _jpeg_size(None) would raise an uncaught AttributeError on this public page.
             poster_abs = storage.safe_abs_path(settings.download_root, asset.poster_path)
-            dims = _jpeg_size(poster_abs) if poster_abs else None
-            if dims:
-                tags.append(_tag("og:image:width", str(dims[0])))
-                tags.append(_tag("og:image:height", str(dims[1])))
+            image_dims = _jpeg_size(poster_abs) if poster_abs else None
+        if image_dims:
+            tags.append(_tag("og:image:width", str(image_dims[0])))
+            tags.append(_tag("og:image:height", str(image_dims[1])))
         tags.append(_tag("twitter:card", "summary_large_image", attr="name"))
         tags.append(_tag("twitter:image", image, attr="name"))
     else: