Rebuild: the merger stops when a pass over the file keys decides nothing

Waiting files counted as work, so the merge loop re-read them for its
whole 30 s budget every minute: Postgres used ~a core even without
workers. Now ~4% at rest, and storing a boneless range costs Postgres
~0.7–1.9 s of CPU (it was 33 s with the loop running). Chunk payloads
are stored EXTERNAL (zstd already; no pglz attempt).

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

database/migrations/1770000000500_create_release_chunks.ts

@@ -24,6 +24,8 @@ export default class extends BaseSchema {
         created_at timestamptz NOT NULL DEFAULT now(),
         PRIMARY KEY (id, created_at)
       ) PARTITION BY RANGE (created_at)${where}`)
+    // The payload is zstd already: no pglz attempt on it.
+    this.schema.raw(`ALTER TABLE release_chunks ALTER COLUMN payload SET STORAGE EXTERNAL`)
     this.schema.raw(`CREATE INDEX release_chunks_release_idx ON release_chunks (release_id)`)
     this.schema.raw(`CREATE INDEX release_chunks_range_idx ON release_chunks (range_id)`)
   }