Feature(backend): Add optional setting to prune queue on startup

[DustyShoe]

Summary

This PR adds an optional setting in Invokeai.yaml:

• Added max_queue_history config to keep the most recent N terminal session-queue items on startup (0 prunes all).
• Startup logic now prunes terminal queue history based on max_queue_history, while preserving pending/staged items.
• Clarified config precedence and log output around queue cleanup.

Previously, the only available setting was clear_queue_on_startup, which wiped the entire session queue (including staged/pending items).

This addition fixes slow startups caused by massive session-queue history buildup, while avoiding loss of staged/pending items.

Tip

To use the new option, add max_queue_history: <N> to invokeai.yaml (or set INVOKEAI_MAX_QUEUE_HISTORY=<N>).

QA Instructions

QA Instructions (skip installation)

1. Set max_queue_history in invokeai.yaml, e.g.:
   • max_queue_history: 5
2. Ensure clear_queue_on_startup: false.
3. Create enough completed/failed/canceled queue items to exceed the limit (e.g., run several quick generations).
4. Restart InvokeAI.
5. Verify:
   • Only the most recent N terminal items remain in history.
   • Pending/staged items are preserved.
   • Logs include: Pruned X completed/failed/canceled queue items (kept up to N).
6. Set max_queue_history: 0, restart, and verify all terminal items are pruned while pending/staged items remain.
7. Set clear_queue_on_startup: true, restart, and verify the entire session queue is cleared (including staged items).

Merge Plan

Versioning/schema

• No schema version bump is required; this is a new optional config setting.
• No DB migration change.
• Docs are generated from InvokeAIAppConfig, so the built docs will include max_queue_history without manual edits.

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)