feat: a creator's trickle becomes one drop instead of a card per message (4390)

The operator, on a feed of "Grouped from 1 Discord message" cards: "the groups
are still single image even when they can clearly be seen as group". 665 of
Yellowroom's 714 drops were one message.

Both join paths demand cosine <= 0.10 to a drop's FIRST image. The stages of
one piece fail that: each is nearest the one before, not the first.
`svtt_wip4` never joined `svtt_wip3` from the day before.

Measured on artist 8 before writing it:
- phash cannot see it. Stages sit 68-134 bits apart; unrelated same-artist
  pairs have a median of 126 and a p5 of 110 (lesson #4400).
- The embedding's nearest neighbour can. Every stage of three real trickles
  had a sibling as its single nearest image in the artist's library. In a
  control over all 137 recent Discord images, a nearest neighbour that was
  another message within 7 days carried the same working name 53 times out
  of 53. Mismatches start past 7 days.

A new merge pass runs last in the sweep. A later drop folds into an earlier
one within discord_group_close_after_hours (168h, the measured 7 days) when
they share a gated leading working name, or when one's image is the other's
nearest neighbour. A drop reaching several earlier drops pulls them all
together, unless two of them are named as different pieces.

A merge carries teaser links across (the payload FK would otherwise cascade
them away). Growth is stamped at the messages' own time, so merging history
never jumps an old drop to the top of the feed. Each drop records the route
it merged by, and is checked once.

Offline replay over Yellowroom's 127 drops since 2025: they become 70 posts.
The Marin trickle ("Very early Marin" -> 3 screenshots -> MarinaraSauce_base)
becomes one post of 5 by nearest neighbour. svtt, 0-k1, cnni14 and 0adm come
together by name.

FAMILY_MAX_POSTS moves to post_naming, so the grouper and the teaser card
share one definition.

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

backend/app/tasks/maintenance.py

@@ -1175,7 +1175,7 @@ def group_discord_drops() -> str:
         return "disabled"
     return (
         f"sources={res['sources']} created={res['posts_created']} "
-        f"joined={res['images_joined']}"
+        f"joined={res['images_joined']} merged={res['drops_merged']}"
     )