Seed batches take file ids from the insert; merge split uploads hourly

SeedService looked up every file of a batch across all files partitions
after inserting it (15% of Postgres' active time); the merge job read
every release on each GC sweep.

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

config/gc.ts

@@ -10,6 +10,9 @@ export default {
   /** How often gc:scheduler recomputes the completeness of releases that changed (LOG.md 2026-10-06 (6)). */
   releaseCompleteIntervalMs: env.get('RELEASE_COMPLETE_INTERVAL_MS', 30 * 60 * 1000),
 
+  /** How often gc:scheduler merges uploads split per poster (D35); it reads every release. */
+  mergeIntervalMs: env.get('GC_MERGE_INTERVAL_MS', 60 * 60 * 1000),
+
   /** Time per sweep for copying old file keys into file_keys until it's complete (LOG.md 2026-10-06 (7)). */
   fileKeysFillMs: env.get('FILE_KEYS_FILL_SECONDS', 90) * 1000,