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/app/queue.py

@@ -21,7 +21,7 @@ QUEUE_INTERACTIVE = "interactive"
 QUEUE_BULK = "bulk"
 
 # Jobs default to the interactive queue; only bulk backfills go to the bulk queue.
-_BULK_JOBS = frozenset({"sync_bootstrap_account"})
+_BULK_JOBS = frozenset({"sync_bootstrap_account", "dedupe_existing_job"})
 
 
 def queue_for(job_name: str) -> str: