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/migrations/versions/0023_attachment_part_index.py

@@ -0,0 +1,36 @@
+"""attachments are kept once: inside their message
+
+An attachment used to be stored twice — inside the raw message blob, and again
+as a blob of its own. Now only the raw message is kept; ``part_index`` says
+which MIME part of it (its position in a walk of the message) holds the
+attachment, so it can be taken out when downloaded. ``blob_sha256`` stays: the
+attachment's content hash, which names its thumbnail and checks what is taken
+out. Rows stored before this have no index; they are found by that hash.
+
+Existing separate copies are removed by a background pass on the worker
+(app/indexer/attachments.py ``dedupe_existing``), only after checking each one
+can be taken out of its message again.
+
+Revision ID: 0023
+Revises: 0022
+Create Date: 2026-10-08
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0023"
+down_revision: str | None = "0022"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE attachments ADD COLUMN part_index integer")
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE attachments DROP COLUMN IF EXISTS part_index")