perf: limit indexed cleanup task batches (#1107)

Limit mail, sent-mail, and indexed address cleanup to configurable batches. Includes E2E coverage and documentation.

vitepress-docs/docs/en/guide/worker-vars.md

@@ -103,6 +103,7 @@
 | `REMOVE_EXCEED_SIZE_ATTACHMENT` | Text/JSON | If attachment exceeds 2MB, remove it, email may lose some information due to parsing                                   | `true`                     |
 | `REMOVE_ALL_ATTACHMENT`         | Text/JSON | Remove all attachments, email may lose some information due to parsing                                                 | `true`                     |
 | `ENABLE_MAIL_GZIP`             | Text/JSON | When enabled, new emails are gzip-compressed and stored in `raw_blob` column to save D1 database space. Existing plaintext `raw` data is automatically compatible for reading. **Run database migration first (`Admin -> Quick Setup -> Database -> Migrate Database` or `POST /admin/db_migration`) to ensure the `raw_blob` column exists before enabling. This feature adds compression/decompression CPU overhead, so enabling it on a paid Cloudflare Worker plan is recommended.** | `true`                     |
+| `CLEANUP_BATCH_SIZE`           | Number    | Per-run limit for mail, sent-mail, and creation/activity-based address cleanup. Defaults to `3000`, valid range `1-5000`. Smaller values reduce per-run D1 pressure; larger values clear backlogs faster | `3000` |
 
 > [!NOTE]
 > Authentication results follow their standards: SPF `none` means no usable domain or SPF record was found, and SPF `neutral` must be treated like `none`; DKIM `none` means the message was unsigned, and DKIM `neutral` is also treated as unsigned; DMARC `none` means no applicable DMARC policy was found. Unregistered results and unsupported method versions are ignored. `JUNK_MAIL_CHECK_LIST` treats these results as absent, while `JUNK_MAIL_FORCE_PASS_LIST` still requires an explicit supported `pass`