Attachments stored once; nightly clean-up of unused files

Attachments are no longer stored apart from their message (they were kept
twice: base64 inside the raw message and decoded on their own). The indexer
records the MIME part (attachments.part_index, migration 0023); downloads,
zip, inline images, thumbnails and forwarding take it out of the message;
videos stream from a short-lived copy (kept a day, 1 GiB per user). A
background pass on worker start removes the copies stored before, each only
after checking it can be taken out again (spec App. D §D.6 item 1, criterion
corrected to <= 60%).

A nightly job (03:47, after the Trash/Spam purge) removes files nothing
refers to any more — deleted or purged messages, previews of attachments
gone, uploads no draft has used for a week, expired streaming copies. Files
younger than an hour are left alone; a sent message waiting for its Sent copy
is held until its job is done. Account removal keeps files a draft still uses.

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

backend/workers/worker.py

@@ -14,6 +14,7 @@ from typing import Any
 
 from arq import func
 
+from app.indexer.attachments import dedupe_existing_job
 from app.outbox.draft_sync import push_draft_to_imap_job, remove_imap_draft_job
 from app.outbox.jobs import file_sent_copy_job, send_job
 from app.push.service import push_new_mail_job
@@ -62,4 +63,8 @@ INTERACTIVE_FUNCTIONS: list[Any] = [
 ]
 
 # Bulk work (§6): long timeout, low concurrency, isolated from interactive jobs.
-BULK_FUNCTIONS = [sync_bootstrap_account]
+BULK_FUNCTIONS = [
+    sync_bootstrap_account,
+    # Resumable (each batch is saved), so a long first run may span timeouts.
+    func(dedupe_existing_job, timeout=6 * 3600),
+]