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/models.py

@@ -278,7 +278,11 @@ class Attachment(Base):
     mime_type: Mapped[str] = mapped_column(Text, nullable=False)
     size_bytes: Mapped[int] = mapped_column(Integer, nullable=False)
     content_id: Mapped[str | None] = mapped_column(Text)
+    # The content's hash (names its thumbnail, checks what is taken out). The
+    # bytes themselves live only inside the message's raw blob, at MIME part
+    # ``part_index`` (NULL for rows stored before that was recorded).
     blob_sha256: Mapped[str] = mapped_column(Text, nullable=False)
+    part_index: Mapped[int | None] = mapped_column(Integer)
 
 
 class ComposeAttachment(Base):