Initialize Scheduler before Migration to ensure hooks are registered

[pfefferle]

Problem

When migration calls add_to_outbox() (e.g., for Move activities), the post_activitypub_add_to_outbox action fires but the scheduler hooks (schedule_outbox_activity_for_federation, schedule_announce_activity) aren't registered yet because:

• Migration::init() runs at init priority 1
• Scheduler::init() runs at init priority 10 (default)

This causes activities added during migration to go to the outbox but never get scheduled for federation.

Proposed Solution

• Change Scheduler::init() priority from 10 (default) to 0, so it runs before Migration::init() (priority 1)
• This ensures that the post_activitypub_add_to_outbox hooks are registered before any migration code calls add_to_outbox()

Testing Instructions

• Apply the changes from Migrate actor IDs from permalink to query-param format #2766 (or any migration that calls add_to_outbox())
• Trigger migration by lowering activitypub_db_version option
• Verify that activities are properly scheduled for federation (check for activitypub_process_outbox cron events)

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GitHub Issue: #2771