fix: the matcher sees a drop's names, finds merged trickles, and catches up on its own (4390, 4392)

Three defects found while answering "is the linking automatic":

1. The name and image checks never worked on a live drop. The corpus keyed
   every image by primary_post_id, but a drop's images belong to its member
   messages and the drop claims them only through provenance, so a drop
   looked nameless and hashless. The tests attached images to the drop
   itself, which discord_grouping never does. Images now count under the
   post a reader sees them on: a message's absorbing drop, else the post
   itself.

2. The trickle merge (e08401c) dates a drop by its first stage, days before
   the release a teaser announces, which put merged trickles outside the 24h
   window. Candidates are now found and timed by their closest member
   message. The sweep follows recently grown drops by their messages' times
   the same way.

3. A pair left pending was skipped forever: the matcher skipped every
   recorded pair, not only decided ones. Pending pairs are now re-scored in
   place and linked once conclusive. Linked and dismissed pairs are still
   never touched.

Also, "Scan now" shared the sweep's 48-hour horizon, so it could not reach
the history it is described as being for. It now scores every post by an
artist with Discord drops (rescan(full=True)).

New tests build drops the way the grouper does, with images owned by the
member messages.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/api/posts.py

@@ -207,6 +207,8 @@ async def rescan_associations():
     be within the window to exist at all); this is the button for a first run
     over a library that predates the feature."""
     async with get_session() as session:
-        result = await association_rescan(session)
+        # full=True: the button reaches the whole history, which the hourly
+        # sweep's 48-hour horizon never does.
+        result = await association_rescan(session, full=True)
         await session.commit()
     return jsonify(result)