Add row-level security cookbook recipe

[javier]

Summary

New cookbook recipe (Advanced SQL): row-level security in QuestDB Enterprise by filtering rows in a view and granting SELECT on the view.

• Simple pattern: one filtered view per trading desk, granted to the matching group.
• Scalable pattern: a master view with an OVERRIDABLE filter plus thin per-desk views that pin the value (non-overridable to callers), so the filter logic lives in one place.
• Explains why callers cannot escape the filter, covers users and service accounts, and adds a posting/covering index note for selective filters.

View and DECLARE OVERRIDABLE behaviour validated against a local instance; GRANT semantics follow the existing Views and RBAC docs.

[github-actions]

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