fix(upload): bound delta-worker connections instead of disabling delta

Follow-up to the connection-exhaustion fix. Rather than routing large files to
plain uploads (which kept STORAGE dedup but gave up delta's re-upload bandwidth
savings), keep delta for every file >= 8 MB and instead cap each worker's
concurrent connections so a few large files uploading at once can't blow past
the browser's ~6-per-host budget and starve the small-file plain uploads.

- deltaWorker.js: serialize negotiate (at most one in flight per worker) and
  drop chunk-PUT concurrency 2 -> 1, so each worker holds ~2 connections max.
- deltaUpload.ts: revert the 64 MB threshold back to 8 MB — every large file
  gets sub-file dedup again. (Storage dedup was never affected: BLAKE3 + CDC +
  ref-counting run server-side for plain and delta uploads alike.)

With main upload concurrency at 2, total in-flight upload connections stay <= ~4.

npm run check: clean, 58 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/api/endpoints/deltaUpload.ts

@@ -13,20 +13,12 @@ import { blake3HexOfFile } from '$lib/vendor/hashWasm';
 
 /** Files smaller than this skip delta: the round-trips cost more than the bytes.
  *  Also the upper bound for client-side whole-file hashing (instant by-hash
- *  uploads) — we never read a file larger than this fully into memory. */
+ *  uploads) — we never read a file larger than this fully into memory. Files at
+ *  or above this run the delta worker (sub-file dedup, saves re-upload
+ *  bandwidth); the worker self-bounds its concurrent connections so a few
+ *  running at once can't exhaust the browser's ~6-per-host budget. */
 export const DELTA_UPLOAD_MIN_SIZE = 8 * 1024 * 1024;
 
-/** Only files at least this large actually run the delta worker. A delta worker
- *  opens SEVERAL concurrent requests (overlapping `negotiate` batches + chunk
- *  PUTs); a few running at once exhaust the browser's ~6 connections-per-host
- *  budget and starve plain uploads (they queue, then the upload watchdog cancels
- *  them — the "stuck at N%" folder upload). Typical large files (e.g. tens of MB)
- *  therefore go through a single-connection plain upload; delta is reserved for
- *  genuinely huge files, where chunked, resumable transfer earns its keep and few
- *  run concurrently. (Delta's real payoff — sub-file dedup — only helps on
- *  re-upload anyway, not the first upload that dominates these batches.) */
-const DELTA_WORKER_MIN_SIZE = 64 * 1024 * 1024;
-
 const DELTA_WORKER_URL = '/workers/deltaWorker.js';
 const DELTA_TIMEOUT_BASE_MS = 120_000;
 const DELTA_TIMEOUT_PER_GB_MS = 90_000;
@@ -72,7 +64,7 @@ export function tryDeltaUpload(
 ): Promise<DeltaUploadAnswer | null> {
 	if (
 		!folderId ||
-		file.size < DELTA_WORKER_MIN_SIZE ||
+		file.size < DELTA_UPLOAD_MIN_SIZE ||
 		usable === false ||
 		typeof Worker === 'undefined'
 	) {