file_keys: fill in the background, use it once complete

Copying all existing keys in the migration read the files rows from the
HDD for over half an hour and held up the deploy. gc:scheduler now copies
them in slices (FILE_KEYS_FILL_SECONDS per sweep) and FileStore checks
every partition until the fill is complete.

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

start/env.ts

@@ -103,6 +103,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   COLLECTION_CONCURRENCY: Env.schema.number.optional(),
   ARCHIVE_WALK_CONCURRENCY: Env.schema.number.optional(),
   RELEASE_COMPLETE_INTERVAL_MS: Env.schema.number.optional(),
+  FILE_KEYS_FILL_SECONDS: 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(),