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>

lib/seed/runner.ts

@@ -1,10 +1,16 @@
-import type { XoverClient, Overview } from '../nntp/xover_client.js'
+import type { Overview } from '../nntp/xover_client.js'
 import { segmentFromHeader, type IngestSegment, type OverviewHeader } from '../ingest/segment.js'
 import { FileAggregator } from '../aggregate/index.js'
 import { toFragment, toSeedFile, type SeedBatch } from './wire.js'
 
+/** Where a range's overviews come from: an NNTP connection, or the seed API's relay. */
+export interface OverviewSource {
+  group(name: string): Promise<unknown>
+  xover(from: number, to: number): Promise<Overview[]>
+}
+
 export interface RunRangeOptions {
-  client: XoverClient
+  client: OverviewSource
   group: string
   /** Inclusive article numbers. */
   start: number