Expose per-device save sync attribution and origin device

[tmgast]

Description

Saves sync per device, but the saves endpoints only ever returned the
caller's own sync row, so a client couldn't tell which devices hold a
save or which one created it. This enhancement exposes full per-device
sync attribution and records each save's creating device.

Changes

• Full device_syncs list. Device-scoped saves responses (list and
  single-save: get, upload, update, download-confirm, track/untrack) now
  return one entry per device that has synced the save, each with its own
  is_current and is_untracked, caller's entry first. A new
  get_syncs_for_saves handler fetches them for a page in one joined
  query. Requests without device_id are unchanged.

• origin_device_id. New nullable column (migration 0082, FK
  devices ON DELETE SET NULL, indexed) recording the device that
  created a save, set on initial upload only and exposed on the save
  schema. Existing saves remain NULL.

Tests

• Multi-device device_syncs content and caller-first ordering;
  device_syncs omitted without device_id.
• origin_device_id set on creation, preserved across updates and
  downloads, nulled when its device is deleted.
• get_syncs_for_saves handler coverage.

---

AI Disclosure
Planning and review assisted by Claude Code.

Checklist

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR adds origin-device attribution for saves and expands save responses to include sync state for all devices (with device names) when a caller provides a device_id.

Changes:

• Add origin_device_id to Save (DB + API schema) with FK behavior ON DELETE SET NULL.
• Add DB query helper to fetch all device syncs for a set of saves, including device names.
• Update saves endpoints to return device_syncs across all devices for device-scoped requests; add/extend tests.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
backend/models/assets.py	Adds origin_device_id column to Save ORM model.
backend/alembic/versions/0081_save_origin_device.py	Introduces migration adding column + index + FK for origin device tracking.
backend/handler/database/device_save_sync_handler.py	Adds get_syncs_for_saves() to fetch syncs + device names in bulk.
backend/endpoints/saves.py	Returns full per-device sync lists (with stable caller-first ordering) and sets origin_device_id on create.
backend/endpoints/responses/assets.py	Exposes origin_device_id on SaveSchema.
backend/tests/handler/database/test_device_save_sync_handler.py	Tests new sync-fetch grouping and origin-device FK cascade behavior.
backend/tests/endpoints/test_saves.py	Tests endpoint behavior for multi-device syncs and origin-device persistence.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.