Relationship: Add configurable limits (per side, optional, active-only)

Problem/Motivation

In the CRM project, each crm_relationship entity connects two contacts — a source and a target.
Currently, there is no way to limit how many relationships of a given type a contact may have.
In real-world use cases, many relationship types have natural limits. For example:

• A child usually has two parents.
• An employee has one primary employer.
• A student may have one advisor, but an advisor can have many students.

Without per-type relationship limits, data entry can produce logically invalid relationships (e.g., a child with five parents).
Site builders need a configurable way to restrict the number of relationships a contact can have per relationship type and per side (source or target).

Steps to reproduce

1. Create a relationship type (e.g., Parent → Child).
2. Add multiple Parent–Child relationships with the same child contact.
3. Observe that there are no limits enforced — any number of parent relationships can be created.