feat(profile): capture the insurer IK number

Add an optional German insurer institution number (IKNR, 9 digits) to
the user profile, alongside the existing insurer name and KVNR. The
IKNR is a public registry identifier for the institution, not a
per-person quasi-identifier, so it is stored plaintext, matching
insurer_name rather than the encrypted KVNR column.

The profile GET echoes the field; PATCH/PUT accept it via Zod —
optional, empty string maps to null, and a non-empty value must be
exactly 9 digits (422 otherwise, no checksum). The write is built
field-by-field. /api/user/profile gains an OpenAPI registration with
the profile read/write schemas.

Migration 0102 adds the nullable column, NULL-defaulted, forward-only.

src/components/settings/__tests__/disable-coach-card.test.tsx

@@ -59,6 +59,7 @@ function buildUser(disableCoach: boolean): AuthUser {
     disableCoach,
     fullName: null,
     insurerName: null,
+    insurerIkNumber: null,
     insuranceNumber: null,
   };
 }

src/components/settings/__tests__/unit-preference-card.test.tsx

@@ -50,6 +50,7 @@ function buildUser(unitPreference: "metric" | "imperial"): AuthUser {
     disableCoach: false,
     fullName: null,
     insurerName: null,
+    insurerIkNumber: null,
     insuranceNumber: null,
   };
 }