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>

app/services/fragment_merger.ts

@@ -12,6 +12,8 @@ export interface MergeResult {
   completed: number
   incomplete: number
   waiting: number
+  /** This round reached the end of the file keys (the next one starts over). */
+  wrapped: boolean
 }
 
 /** File keys per round. */
@@ -29,14 +31,15 @@ class FragmentMerger {
   private cursor = ''
 
   async run(): Promise<MergeResult> {
-    const result: MergeResult = { files: 0, completed: 0, incomplete: 0, waiting: 0 }
+    const result: MergeResult = { files: 0, completed: 0, incomplete: 0, waiting: 0, wrapped: false }
     const { rows: keys } = await db.rawQuery(
       `SELECT DISTINCT f.file_key FROM fragments f JOIN ranges r ON r.id = f.range_id
        WHERE r.status = 'done' AND f.file_key > ? ORDER BY f.file_key LIMIT ?`,
       [this.cursor, BATCH]
     )
     // At the end of the keys: the next round starts over.
-    this.cursor = keys.length < BATCH ? '' : keys.at(-1).file_key
+    result.wrapped = keys.length < BATCH
+    this.cursor = result.wrapped ? '' : keys.at(-1).file_key
     if (keys.length === 0) return result
     const rules = new RuleSet(await RuleSets.current())
     const rows = await db