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/indexer/indexer.py

@@ -216,8 +216,9 @@ async def persist_message(
             )
         )
 
+    # Attachments are not stored apart: they stay inside the raw blob, and are
+    # taken out of it when wanted (app/indexer/attachments.py).
     for att in parsed.attachments:
-        att_sha = blobstore.store(user_id, att.payload)
         session.add(
             Attachment(
                 message_id=message.id,
@@ -226,7 +227,8 @@ async def persist_message(
                 mime_type=att.mime_type,
                 size_bytes=len(att.payload),
                 content_id=att.content_id,
-                blob_sha256=att_sha,
+                blob_sha256=blobstore.sha256_hex(att.payload),
+                part_index=att.part_index,
             )
         )