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/app/api/auth/me/route.ts

@@ -66,6 +66,7 @@ export const GET = apiHandler(async () => {
     // v1.7.0 — health-record export identity fields. All optional.
     fullName: user.fullName ?? null,
     insurerName: user.insurerName ?? null,
+    insurerIkNumber: user.insurerIkNumber ?? null,
     insuranceNumber,
   });
 });

src/app/api/auth/profile/route.ts

@@ -33,6 +33,7 @@ export const PUT = apiHandler(async (request: NextRequest) => {
     timezone: result.user.timezone,
     fullName: result.user.fullName,
     insurerName: result.user.insurerName,
+    insurerIkNumber: result.user.insurerIkNumber,
     hasInsuranceNumber: result.user.hasInsuranceNumber,
   });
 });