feat(migration): V22 add title, person_type, nullable first_name

- Add title VARCHAR(50) column
- Add person_type VARCHAR(20) NOT NULL DEFAULT 'PERSON' with CHECK
  constraint (PERSON, INSTITUTION, GROUP, UNKNOWN — SKIP excluded)
- Drop NOT NULL on first_name for non-person entities

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V22__person_type_title_nullable_firstname.sql

@@ -0,0 +1,12 @@
+-- Add title column for honorifics/salutations (Dr., Tante, Frau, etc.)
+ALTER TABLE persons ADD COLUMN title VARCHAR(50);
+
+-- Add person_type column to distinguish persons from institutions/groups.
+-- SKIP is intentionally omitted: it exists in the Java enum for parse-time
+-- filtering but must never be persisted. The CHECK constraint enforces this.
+ALTER TABLE persons ADD COLUMN person_type VARCHAR(20) NOT NULL DEFAULT 'PERSON';
+ALTER TABLE persons ADD CONSTRAINT chk_person_type
+    CHECK (person_type IN ('PERSON', 'INSTITUTION', 'GROUP', 'UNKNOWN'));
+
+-- Make first_name nullable for non-person entities (institutions, groups)
+ALTER TABLE persons ALTER COLUMN first_name DROP NOT NULL;