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/hooks/use-auth.ts

@@ -53,6 +53,11 @@ export interface AuthUser {
    */
   fullName: string | null;
   insurerName: string | null;
+  /**
+   * v1.8.6 — optional German insurer institution number (IKNR, 9 digits).
+   * Surfaced on the FHIR `Coverage` resource's payor Organization.
+   */
+  insurerIkNumber: string | null;
   insuranceNumber: string | null;
 }