Contact search/filter endpoint missing - users cannot find contacts by organisation name

[kaiwend]

Bug Description

The contacts API only supports listing all contacts with basic pagination (skip/limit),
but there is no way to search or filter contacts by organisation name. As the number of
contacts grows, users have to scroll through all of them to find what they need.

Current Behavior

GET /api/v1/contacts/?skip=0&limit=100 returns all contacts without any filtering capability.

Expected Behavior

Users should be able to filter contacts by organisation name, e.g.:
GET /api/v1/contacts/?search=Acme

This should perform a case-insensitive partial match on the organisation field.

Acceptance Criteria

• Add an optional search query parameter to GET /api/v1/contacts/
• Filter should be case-insensitive and support partial matches
• Search should apply alongside existing skip/limit pagination
• The count in the response should reflect the filtered total, not the overall total
• Add a test for the new search functionality