seed:node spreads its ranges and inspections over worker processes

One process ran everything on one core. SEED_PROCESSES (default: the
number of cores) worker processes now each run seed:node with a share of
the slots; stop signals are passed on and crashed workers restarted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

seed-node/README.md

@@ -64,6 +64,7 @@ for the same name replaces the token; `--disable` revokes it.
 | `SEED_TOKEN` | the token from step 1 |
 | `SEED_RANGES` | ranges seeded at once. Start with 2–4; `0` for no seeding. |
 | `SEED_INSPECTORS` | inspections and archive walks at once (default `0`). Try 2–4 on a machine with spare cores. |
+| `SEED_PROCESSES` | optional: how many processes the ranges and inspections are spread over, one CPU core each. Default: the number of cores (never more than ranges + inspections). Lower it to leave cores free. |
 | `SEED_FETCH=direct` and `NNTP_*` | optional: seed with this node's own NNTP account (see the commented lines). `NNTP_TLS_SERVERNAME` only through a tunnel: the news server's real hostname, for the TLS certificate check. |
 
 Nothing else: the database, Redis and web server settings the indexer