Follow+Notes: Expose ability to follow a grant via Finder API

[TylerHendrickson]

Subtask of [STORY]: Update 'Status' to 'Follow + Note' feature #2960

Blocked by

• Follow+Notes: Add grant followers management to collaboration library #3203

Blocks

TBD

Definition of Done

A new API route exists that allows users to follow a specific grant.

Implementation Details

In packages/server/src/routes/grants.js, define a new PUT /:grantId/follow API route handler as follows:

• Requires authentication.
• Calls followGrant() from packages/server/src/lib/grantsCollaboration with the grant ID from request path parameters and the authenticated user ID.