Split knowledge_graph feature flag for per-user rollout

What does this MR do and why?

Splits the :knowledge_graph feature flag into two flags to fix mixed actor types and enable per-user rollout.

Problem

The :knowledge_graph flag was used with two different actor types:

• :instance — for infrastructure (event store subscriptions, internal/admin APIs)
• current_user — for user-facing access (dashboard, sidebar, GraphQL, REST API)

Mixing actor types on a single flag prevents per-user rollout via chatops. Enabling globally (for infra) also enables the UI for everyone, and enabling per-user doesn't activate the infrastructure.

Solution

Split into two flags:

Flag	Type	Actor	Purpose
:knowledge_graph	beta	current_user	Gates user-facing Orbit UI and API access. Supports gradual per-user rollout via chatops.
:knowledge_graph_infra	ops	:instance	Gates background event subscriptions (cache expiry workers) and internal/admin APIs.

Rollout strategy

1. Enable :knowledge_graph_infra at the instance level on staging
2. Gradually roll out :knowledge_graph to users via chatops

References

• Related MR: !225813 (merged), depends on !225997 (merged)
• Reviewer discussion: !225813 (comment 3132415114)
• Epic: &19744
• Feature flag docs: https://docs.gitlab.com/development/feature_flags/#mixing-actor-types