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>

app/services/SeedOrchestrator.ts

@@ -3,6 +3,7 @@ import db from '@adonisjs/lucid/services/db'
 import NntpService from '#services/NntpService'
 import SeedService from '#services/SeedService'
 import BackfillService from '#services/BackfillService'
+import SeedFetchPool from '#services/SeedFetchPool'
 import { articleAtDate } from '#lib/nntp/date_search'
 import { normalizeSeedFile, type SeedBatch } from '#lib/seed/wire'
 
@@ -92,6 +93,19 @@ class SeedOrchestrator {
     return stored
   }
 
+  /**
+   * Relay: overviews of `from`..`to` of a leased range, fetched here and handed
+   * on as the gzip stream the news server sent. Keeps the lease alive.
+   */
+  async chunk(nodeId: number, rangeId: number, from: number, to: number): Promise<Buffer> {
+    const range = await this.leased(nodeId, rangeId)
+    const start = Math.max(from, Number(range.start_id))
+    const end = Math.min(to, Number(range.end_id), start + 49_999)
+    if (end < start) throw new RangeError(`Articles ${from}–${to} are outside range ${rangeId}`)
+    await db.from('seed_ranges').where('id', rangeId).update({ heartbeat_at: new Date() })
+    return SeedFetchPool.fetch(range.group_name, start, end)
+  }
+
   async finish(nodeId: number, rangeId: number) {
     await this.leased(nodeId, rangeId)
     await db.from('seed_ranges').where('id', rangeId).update({ status: 'done', done_at: new Date(), heartbeat_at: new Date(), error: null })