fix: rank date sources instead of letting scan order decide

The previous commit had this backwards: the sidecar date was only
consulted when the existing date came from an mtime, so a filename date
silently outranked what Instagram itself reported.

The order is sidecar, then filename, then mtime -- metadata first,
mtime last, since mtime is when the file hit disk and says nothing about
when the post was made. Ties keep the incumbent so two equally
authoritative files cannot flip a post's date by scan order.

Extracted to src/lib/post-dates.ts rather than left inline, because the
rule is easy to state and easy to get wrong -- the tests include an
order-independence case that would have caught the original mistake.

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

CLAUDE.md

@@ -74,7 +74,7 @@ Three different JSON shapes turn up as `.json`, so they are told apart structura
 
 The gallery-dl sidecar is the only source that states what a post *is*: its `type` (`post` / `reel` / `story` / `highlight`) is Instagram's own classification, so `post.isReel` set from it beats every fallback in `post-tabs.ts`. This matters — of the 781 items in `official_band - reels`, the sidecars say only **360 are reels**; the other 421 are ordinary feed videos the clips endpoint returns via `include_feed_video`. Directory-based classification counted all 781.
 
-**Dates: a real date always beats an mtime.** Only JDownloader highlights lack a date in the filename, and `parseArchiveFilename` marks those with `dateFromMtime` so the scanner can upgrade them when the same item also appears under a dated name. Without it the date depended on which file the scan happened to reach first.
+**Dates are ranked, not last-write-wins** (`src/lib/post-dates.ts`): sidecar (what Instagram reported) beats filename (what the fetcher wrote) beats mtime (when the file hit disk, and unrelated to when it was posted). Ties keep the incumbent. Several files describe one post and they are scanned in directory order, not in order of trustworthiness, so without the ranking the date was decided by whichever file came first. Only JDownloader highlights fall to mtime at all — `parseArchiveFilename` flags those via `dateFromMtime`.
 
 ### Cache and local-archive persistence (`src/lib/archive-cache.ts`)