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>

seed-node/README.md

@@ -1,10 +1,18 @@
 # Seeding node
 
 A seeding node indexes old article ranges for the indexer (CONCEPT.md D37,
-D38). It asks the seed API for a range over HTTPS, fetches that range's
-headers over its own NNTP connections, groups them into files, and posts the
+D38). It asks the seed API for a range over HTTPS, gets that range's headers
+(by default through the seed API, which fetches them from the news server and
+passes them on gzip-compressed), groups them into files, and posts the
 results back. It only makes outgoing connections: no port forwarding,
-database or Redis.
+database, Redis or news server account. Only a token and
+`SEED_CONNECTIONS` need setting.
+
+Headers through the seed API (`SEED_FETCH=relay`, the default) use the
+indexer's news server account: `SEED_NNTP_CONNECTIONS` in the indexer's
+`.env` sets how many connections it fetches with for all nodes together.
+With `SEED_FETCH=direct` a node uses its own NNTP account instead (fill in
+the NNTP lines in `node.env`).
 
 ## What a node needs