feat: read gallery-dl sidecars for reel type and post dates

The .json sidecars published with the ARTMS fetch were inert: the scanner
fed them through the Instaloader path, where `node.edge_media_to_caption`
and `checkIsStory`'s `product_type` are both absent, so nothing happened.

They are now recognised structurally -- flat, with post_shortcode and
type, and none of the markers the other two JSON shapes carry -- and used
for three things:

- `type` sets post.isReel, which post-tabs prefers over every fallback.
  This is Instagram's own classification and it disagrees with ours a
  lot: of 781 items in "official_artms - reels", the sidecars say only
  360 are reels. The other 421 are feed videos the clips endpoint returns
  via include_feed_video, and the directory-based rule counted them all.
- `description` fills the caption where no .txt exists.
- `date` dates a post whose filename could not.

Also fixes date precedence. Only JDownloader highlights lack a date in
the filename, so parseArchiveFilename now marks those as mtime-derived
and the scanner lets any real date replace them -- previously the date
depended on which file the scan reached first.

Verified against real published files: a directory of three type=post and
three type=reel renders 6 in the grid and exactly the 3 reels in the
Reels tab.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lib/post-tabs.ts

@@ -41,11 +41,16 @@ export const hasReelSource = (posts: Post[]): boolean => posts.some(p => p.sourc
  * or an old IGTV upload, all three of which are plain `GraphVideo` nodes
  * distinguished only by `product_type`.
  */
-export const makeIsReel = (posts: Post[]): ((post: Post) => boolean) => (
-  hasReelSource(posts)
+export const makeIsReel = (posts: Post[]): ((post: Post) => boolean) => {
+  const guess = hasReelSource(posts)
     ? (post: Post) => post.source === 'reels'
-    : (post: Post) => post.media.length === 1 && post.media[0]?.type === 'video'
-);
+    : (post: Post) => post.media.length === 1 && post.media[0]?.type === 'video';
+
+  // `isReel` comes from a gallery-dl sidecar and is Instagram's own answer, so
+  // it beats both fallbacks — per post, since an archive is usually a mix of
+  // files fetched before and after sidecars existed.
+  return (post: Post) => post.isReel ?? guess(post);
+};
 
 /** Preference order when the same post was fetched into more than one directory. */
 const SOURCE_RANK: Record<SourceKind, number> = { reels: 0, posts: 1, stories: 2, highlight: 3 };