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/types/index.ts

@@ -37,6 +37,12 @@ export interface Post {
   isStory?: boolean;
   /** Defaults to 'posts' for archives without sidecar directories. */
   source?: SourceKind;
+  /**
+   * Instagram's own answer to "is this a reel", from a gallery-dl `.json`
+   * sidecar. Undefined when the archive carries no such sidecar, which is when
+   * the viewer has to fall back to guessing — see src/lib/post-tabs.ts.
+   */
+  isReel?: boolean;
   /** Highlight this post belongs to, for source === 'highlight'. */
   highlightTitle?: string;
 }