feat(grobid): run GROBID on-demand (restart "no") to free ~3.5GB when idle

GROBID is only needed during reference extraction (ra_grobid_backfill.py),
not for normal corpus operations (search, embeddings, wiki). Idle it still
holds ~3.5GB on the 8GB Jetson, so keep it stopped by default.

- infra/docker-compose.yml: grobid restart unless-stopped -> "no" (no auto-start
  on boot; db keeps unless-stopped). Start/stop papers-grobid around an R-A run.
- docs/infra/grobid-jetson.md: add "Run on-demand" section with the start/stop
  workflow, and record the refextract head-to-head evaluation on the Jetson that
  led to keeping GROBID: far worse DOI recall on this math corpus (lutz thesis
  101 usable IDs vs refextract 2) and slower per PDF despite a smaller footprint.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker-compose.yml

@@ -44,7 +44,11 @@ services:
     init: true            # reap zombie children (GROBID docs recommend --init)
     ulimits:
       core: 0             # disable core dumps (GROBID docs: --ulimit core=0)
-    restart: unless-stopped
+    # On-demand only: GROBID is needed solely during reference extraction
+    # (scripts/ra_grobid_backfill.py), not for normal corpus operations — start it
+    # for an R-A run, stop it after to reclaim ~3.5GB. "no" = no auto-start on boot.
+    # See docs/infra/grobid-jetson.md ("Run on-demand").
+    restart: "no"
     # GROBID is RAM-hungry; cap usage on the constrained Jetson (Orin Nano, 8GB
     # RAM shared with Postgres). 3GB suffices for references; 4g leaves headroom.
     # Honored by `docker compose up` under Compose v2 (no swarm needed).