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>

start/env.ts

@@ -106,6 +106,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   RELEASE_COMPLETE_INTERVAL_MS: Env.schema.number.optional(),
   FILE_KEYS_FILL_SECONDS: Env.schema.number.optional(),
   GC_AWAIT_INSPECTION_SECONDS: Env.schema.number.optional(),
+  GC_MERGE_INTERVAL_MS: Env.schema.number.optional(),
   ARCHIVE_WALK_MAX_SEGMENTS: Env.schema.number.optional(),
   DEFERRED_SETTLE_MS: Env.schema.number.optional(),
   DEFERRED_EXPAND_MAX_PARTS: Env.schema.number.optional(),