GET /users/me/calibrations endpoint

[bencap]

Add a new authenticated endpoint that returns all score calibrations created by the current user. This allows users to view and manage their own calibrations from a single endpoint, similar to the existing /users/me/collections and /users/me/access-keys patterns.

Motivation
Currently, there is no way for a user to retrieve all calibrations they have created across different score sets. Users must know the specific score set URN and query each one individually. A /me calibrations endpoint provides a unified view of a user's calibration work.

Proposed Endpoint
Route: GET /api/v1/users/me/calibrations
Auth: require_current_user (401 if unauthenticated)
Response model: list[ScoreCalibrationWithScoreSetUrn]
Behavior: Returns all ScoreCalibration records where created_by_id matches the authenticated user's ID. Eager-loads the score set relationship (with contributors) so the score_set_urn field populates correctly.

Test Plan
Unauthenticated user receives 401
Authenticated user with no calibrations receives empty list
Authenticated user sees only their own calibrations (not others')
Multiple calibrations across different score sets are all returned