Seed relay: nodes get their headers through the seed API

The seed API keeps a pool of NNTP connections (SEED_NNTP_CONNECTIONS) and
serves GET /api/v1/seed/ranges/:id/chunk?from=&to= for leased ranges:
the gzip stream the news server sent, passed on unopened. seed:node fetches
that way by default (SEED_FETCH=relay), so a node needs only a token and
SEED_CONNECTIONS; SEED_FETCH=direct keeps its own NNTP connections. All
fetching then comes from the indexer's IP (the account allows two).

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

config/indexer.ts

@@ -63,6 +63,11 @@ export default {
    * Header backfill (`group:backfill`, CONCEPT.md D32): low-priority,
    * header-only fetch ranges walking back through a group's history.
    */
+  seed: {
+    /** NNTP connections the seed API fetches with for relay nodes (CONCEPT.md D38); 0: relay off. */
+    nntpConnections: env.get('SEED_NNTP_CONNECTIONS', 0),
+  },
+
   backfill: {
     /** Backfill ranges waiting or running at a time, over all groups. */
     maxQueued: env.get('BACKFILL_MAX_QUEUED', 40),