fix(canvas): make multi-chunk pixel writes atomic and refund cooldown on failure

- wrap writePixels in state.storage.transactionSync so a partial multi-chunk
  failure doesn't leave the canvas half-written or out of sync with the WS
  broadcast
- size new chunk buffer against chunkSize(chunkId) instead of the persisted
  blob's length so writes after a canvas-grow no longer silently drop OOB
  bytes in the formerly-last short chunk
- refund the cooldown row when writePixels throws so transient storage
  errors stop soft-DOSing the user (and halving image-uploader throughput)
- bound readAllChunks by chunk_id < CHUNK_COUNT and trim oversized blobs so
  orphan rows from a future shrink no longer crash GET /api/canvas
- require a positive Content-Length on /api/place (411) and reject above the
  pre-parse cap (413); previously a missing or zero header bypassed the cap
- drop String(err) from the 500 response body
- drain the INSERT cursor symmetrically with the UPDATE branch in tryAcquire
- assert CHUNK_BYTES <= 2 MB at module load (DO SQLite per-cell BLOB cap)
- correct the inverted webSocketClose comment and guard the re-close call
- add tests for missing / zero / oversized Content-Length

Plan: plans/260510-0232-fix-do-migration-followups/phase-01-do-storage-atomicity.md

src/worker.js

@@ -21,7 +21,13 @@ app.get('/api/canvas', async (c) => {
 
 /** POST /api/place — validate at the edge, forward to the DO. */
 app.post('/api/place', async (c) => {
-  const contentLength = parseInt(c.req.header('content-length') || '0', 10);
+  // Require a positive Content-Length. Missing or zero would otherwise let a
+  // chunked-transfer-encoded body bypass the MAX_BODY_BYTES pre-parse cap.
+  const contentLengthRaw = c.req.header('content-length');
+  const contentLength = parseInt(contentLengthRaw ?? '', 10);
+  if (!Number.isFinite(contentLength) || contentLength <= 0) {
+    return c.json({ error: 'content_length_required' }, 411);
+  }
   if (contentLength > MAX_BODY_BYTES) {
     return c.json({ error: 'body_too_large', max: MAX_BODY_BYTES }, 413);
   }