feat: FC authors the post that Discord never wrote (milestone 388 step E2)

Discord is a delivery channel, not a publisher. One message is not one post,
and today every message lands as its own `post` row, so chat lines compete
with authored work for the same surface. Rather than demote them into a
second-class feed, FC now writes the post itself: one row per DROP, its
images the drop's images, its body the messages' text in arrival order.

Synthesising a `Post` (rather than inventing a parallel entity) is the whole
point — the result is post-shaped by construction, so feed, provenance,
translation, attachments and series keep working on it unchanged.

The predicate is three axes ANDed, and the time one does the real work:

    same source  AND  cosine distance <= threshold  AND  no gap > window

Similarity alone over-groups, and that is the failure that would make this
useless: any two pieces of the same character by the same artist sit close in
SigLIP space, so a cosine-only rule collapses a month of one character into a
single "post". Two details inside the predicate are load-bearing —

* distance is measured to the group's SEED, never to the previous member,
  because chaining lets a group DRIFT: twenty small steps walk from one piece
  to a completely different one, each hop individually within threshold;
* the window is measured between CONSECUTIVE messages, not from the first, so
  an artist trickling variants out over an evening stays one drop.

Why a post-import sweep and not part of ingest. The obvious alternative was to
migrate Discord to the native post-first ingester (#1266) and group at capture
time. That cannot work: the grouping signal is `siglip_embedding`, which is
produced asynchronously AFTER import (tasks/ml.py, the GPU backfill), so at
capture time there is nothing to group on. Grouping is necessarily something
that happens once the vectors catch up — hence a re-runnable sweep that skips
what it cannot yet place, and an hourly (not daily) cadence.

The honesty rule, enforced in the schema. `post.synthesized_by` names the
grouper; `synthesis_details` records the members, the count, and the
thresholds AS THEY WERE (they are operator-tunable, so without that "why did
it group these" is unanswerable a month later). Member posts are absorbed, not
destroyed — they remain the images' true origin and the audit trail — and
`absorbed_by_post_id` is ON DELETE SET NULL, so deleting a synthetic post
releases its members back into the feed in one DELETE with no repair step.
`post_title` stays NULL deliberately: a synthesised title is the one place
this could put words in a creator's mouth.

Two guards the first draft would have failed:

* the per-run cap took the lowest post IDs, not the oldest posts — DISTINCT ON
  forces its own ORDER BY, so the sort now happens outside the subquery;
* a cap landing mid-drop would have published a truncated group claiming to be
  a whole drop, so the last group is left for the next run.

And one vacuous test caught before it shipped: the support vector perturbed a
single component of an all-ones vector, moving it ~1e-6, so every distance
assertion passed regardless of what the predicate did. `_vec` now builds a
unit vector at a stated angle, where distance is exactly 1 - cos(delta) —
rule 167, a guard has to be able to fail.

UI (rule 27) follows in the next commit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LNXXULQDjVZmbuNa2G9mD9

backend/app/tasks/maintenance.py

@@ -1131,3 +1131,40 @@ def vacuum_analyze() -> dict:
             done.append(table)
     log.info("vacuum_analyze complete: %s", done)
     return {"vacuumed": done}
+
+
+@celery.task(
+    name="backend.app.tasks.maintenance.group_discord_drops",
+    soft_time_limit=1800, time_limit=2100,
+)
+def group_discord_drops() -> str:
+    """Milestone 388 E2: group Discord message-posts into the drops FC authors.
+
+    Lives on the MAINTENANCE lane, not the ml lane, even though it reads SigLIP
+    vectors — it does no inference and imports no ML library, and the ml-worker
+    is an OPTIONAL container (B3). Routing it to 'ml' would silently disable
+    grouping on every stack that runs a GPU agent and drops that container,
+    which is the same trap gpu_queue.py was moved here to avoid.
+
+    Async body under its own loop, per the _async_session contract: the sweep
+    needs pgvector column reads and the shared services are async.
+    """
+    import asyncio
+
+    from ..services.discord_grouping import sweep
+    from ._async_session import async_session_factory
+
+    async def _run() -> dict:
+        async_factory, engine = async_session_factory()
+        try:
+            async with async_factory() as session:
+                result = await sweep(session)
+                await session.commit()
+            return result
+        finally:
+            await engine.dispose()
+
+    res = asyncio.run(_run())
+    if not res["enabled"]:
+        return "disabled"
+    return f"sources={res['sources']} created={res['posts_created']}"