[feat] Round robin job scheduling in multiuser mode

[lstein]

In multiuser mode, a single user could monopolize the queue by enqueueing large batches, forcing other users to wait indefinitely. This adds a round_robin queue mode that interleaves jobs across users so each gets a turn before any user gets a second slot.

Changes

• New config field session_queue_mode ("FIFO" | "round_robin", default "round_robin"): controls dequeue ordering. Configurable via invokeai.yaml, env var (INVOKEAI_SESSION_QUEUE_MODE), or CLI.
• Single-user mode always uses FIFO — session_queue_mode is ignored when multiuser=False.
• Round-robin dequeue() SQL: uses two CTEs — user_last_served tracks MAX(started_at) per user; user_next_item selects each user's best pending item (priority DESC, item_id ASC). Rows are ordered by COALESCE(last_served_at, '1970-01-01') ASC so the least-recently-served user always goes next.
• Tests: 10 new tests covering FIFO and round-robin behavior, including the exact interleaving example from the issue:

Queued	Processed
A1, A2, B1, C1, C2, A3	A1, B1, C1, A2, C2, A3

QA Instructions

1. Run with multiuser: true in invokeai.yaml (default session_queue_mode: round_robin).
2. Enqueue several batches as two different users — confirm jobs alternate per user rather than draining one user's queue fully before moving to the next.
3. Set session_queue_mode: FIFO and confirm strict insertion-order is restored.
4. Run with multiuser: false — confirm FIFO is used regardless of session_queue_mode.

Run the new unit tests:

pytest tests/app/services/session_queue/test_session_queue_dequeue.py -v

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)