feat: add Instagram field to organisation model

[james-cross]

Summary

• Adds Instagram optional string field to organisation type, Mongoose model, and Zod validation schema
• Validated as a URL, consistent with existing social media fields (Facebook, Twitter, Bluesky)
• Field already exists in some database documents but was not supported by the API

Test plan

• Create an organisation with an Instagram URL and verify it persists
• Update an existing organisation to add/remove Instagram and verify changes save
• Verify invalid URLs are rejected by validation