feat: placement reconciler tasks + API (4246, slice 3b)

Three Celery tasks wrapping the 3a service, and the endpoints that drive
them. Routed to `maintenance_long` alongside backups: 33k renames on NFS have
no business in the quick lane where the self-healing sweeps live (the
2026-06-07 starvation).

A durability bug in 3a, found by thinking about what a crash costs rather
than by a failing test: `apply_run` wrote its ledger only at the end, so a
worker dying at row 30,000 of 33,789 would have taken the undo information
for the first 29,999 with it — and that ledger is the ONLY record of where
those files came from. It now persists every 200 moves. Two things fell out
of writing that:

- `_persist` reassigns `run.moves`, so the loop had to snapshot the plan
  first rather than iterate the attribute it rewrites.
- the reassignment is itself load-bearing: SQLAlchemy does not track in-place
  mutation of a JSONB list, so an `.append()` alone would never reach the
  database and the ledger would have stayed silently empty.

Re-running a partially-applied plan is safe — the moved rows no longer match
their `from` and refuse as "row moved since planning" — but `apply_placement`
deliberately has NO autoretry: re-entering a half-applied plan should be the
operator's call after reading what happened, not the queue's.

Endpoints gate on run state as well as the service does, so a stray POST
cannot re-apply an applied run. The list response omits `moves` (an applied
whole-library run carries tens of thousands of entries); the detail endpoint
includes them, because that detail IS the preview read before agreeing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/celery_app.py

@@ -35,6 +35,7 @@ def make_celery() -> Celery:
             "backend.app.tasks.backup",
             "backend.app.tasks.admin",
             "backend.app.tasks.library_audit",
+            "backend.app.tasks.library_placement",
             "backend.app.tasks.translation",
         ],
     )
@@ -62,6 +63,8 @@ def make_celery() -> Celery:
             # 2026-06-07: a 2h audit blocked vacuum/backup/normalize for hours).
             "backend.app.tasks.maintenance.*": {"queue": "maintenance"},
             "backend.app.tasks.backup.*": {"queue": "maintenance_long"},
+            # 33k renames on NFS: long lane, same as backups.
+            "backend.app.tasks.library_placement.*": {"queue": "maintenance_long"},
             "backend.app.tasks.admin.*": {"queue": "maintenance_long"},
             "backend.app.tasks.library_audit.*": {"queue": "maintenance_long"},
             # Translation backfill hits the LLM (~1–6s/item) → the long lane so it