Create knowledge graph service/interface

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Create a knowledge graph service/interface on Rails side which:

• by "interface" is not meant API but rather a thin library/service which can be used by other callers in Rails
• accepts a user, a project and a cypher query for graph DB
• handles authorization (verifies that the user can use the graph DB for the project)
• finds "primary" node (first replica) for the project's graph DB
• sends query to the graph node and processes response
• does rate-limiting per-user (related to https://gitlab.com/gitlab-com/gl-security/product-security/infrastructure-security/bau/-/issues/15505#note_2635336576) - implement separate rate limiting for AI agents vs human users. Consider per-project quotas and sliding window rate limits. Add metrics to monitor AI agent usage patterns