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/api/compose.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import asyncio
 import uuid
 from datetime import UTC, datetime, timedelta
 from typing import Annotated
@@ -15,6 +16,7 @@ from app.api.schemas import AttachmentOut
 from app.config import get_settings
 from app.db import get_session
 from app.identity.deps import CurrentUser, require_csrf
+from app.indexer import attachments as att_store
 from app.indexer import blobstore
 from app.models import Attachment, BodyPart, ComposeAttachment, Draft, Mailbox, Message, Outbox
 from app.outbox.jobs import RETRY_WINDOW
@@ -207,6 +209,11 @@ async def adopt_remote_draft(message_id: uuid.UUID, user: CurrentUser, db: DbSes
     )
     attachment_ids: list[str] = []
     for a in att_rows:
+        # Kept only inside the message: give the compose attachment a blob of its own.
+        if not await asyncio.to_thread(
+            att_store.materialize, user.id, msg.blob_sha256, a.blob_sha256, a.part_index
+        ):
+            continue
         ca = ComposeAttachment(
             user_id=user.id,
             filename=a.filename,