Add automatic reviewer assignment setting, worker, and trigger wiring

Summary

• Adds AutoAssignReviewersWorker (idempotent, data_consistency :sticky)
• Triggers auto-assignment from SyncCodeOwnerApprovalRules service (covers both sync and async code owner rule sync paths)
• Triggers auto-assignment on MR draft-to-ready transition
• Adds "Automatic reviewer assignment" toggle to MR settings page (Beta)
• Setting gated behind auto_assign_code_owner_reviewers feature flag + code_owners license
• Preserves existing reviewers when assigning (never overwrites, only appends)
• Compatible with Duo Code Review: if only the Duo bot is assigned, code owners are still added alongside
• Naming is intentionally generic to support future assignment strategies (e.g. DAP) beyond code owners

Part 3 of 3 for CODEOWNERS automatic reviewer assignment. Feature is live behind auto_assign_code_owner_reviewers flag + project setting after this merges.

Related

• Issue: #589700
• Part 1 (DB): !224175 (merged)
• Part 2 (Service): !224176 (merged)

Settings UI

The new setting appears on Settings > Merge Requests as a dedicated section:

• Automatic reviewer assignment (section heading)
• Checkbox: "Automatically assign code owners as reviewers" (Beta badge)
• Help text: "Code owners are automatically assigned as reviewers when a merge request is marked as ready. Draft merge requests are excluded."
• Only visible when auto_assign_code_owner_reviewers feature flag is enabled and code_owners license is available