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/archive-patterns.test.ts

@@ -10,6 +10,7 @@ describe('parseArchiveFilename — Instagram export format', () => {
       index: 1,
       ext: 'mp4',
       isStory: false,
+      dateFromMtime: false,
     });
   });
 
@@ -156,3 +157,35 @@ describe('gallery-dl / JDownloader naming interop', () => {
     expect(dated.date).toBe('2024-08-04');
   });
 });
+
+/**
+ * Highlights are the only files with no date in the name, so they fall back to
+ * mtime — which is when the file was written, not when it was posted. Callers
+ * need to know the difference to let a real date win.
+ */
+describe('dateFromMtime', () => {
+  const mtime = Date.parse('2026-08-17T00:00:00Z');
+
+  it('flags an undated highlight name as mtime-dated', () => {
+    const p = parseArchiveFilename('4utumn07 - C-IImhvpFuk.jpg', 'highlight', mtime)!;
+    expect(p.date).toBe('2026-08-17');
+    expect(p.dateFromMtime).toBe(true);
+  });
+
+  it('does not flag a highlight that carries its own date', () => {
+    const p = parseArchiveFilename('2024-08-04_4utumn07 - C-IImhvpFuk.jpg', 'highlight', mtime)!;
+    expect(p.date).toBe('2024-08-04');
+    expect(p.dateFromMtime).toBe(false);
+  });
+
+  it('never flags ordinary post or Instaloader names', () => {
+    expect(parseArchiveFilename('2023-04-19_u - ABC.mp4', 'posts', mtime)!.dateFromMtime).toBe(false);
+    expect(parseArchiveFilename('2024-01-01_12-00-00_UTC.jpg', 'posts', mtime)!.dateFromMtime).toBe(false);
+  });
+
+  it('leaves the date empty rather than guessing when no mtime is given', () => {
+    const p = parseArchiveFilename('4utumn07 - C-IImhvpFuk.jpg', 'highlight')!;
+    expect(p.date).toBe('');
+    expect(p.dateFromMtime).toBe(false);
+  });
+});