feat(model): add PersonType enum and MAIDEN_NAME alias type

PersonType has 5 values: PERSON, INSTITUTION, GROUP, UNKNOWN, SKIP.
SKIP is intentionally excluded from the DB CHECK constraint (added
in migration) as defense-in-depth. MAIDEN_NAME added to
PersonNameAliasType for #209.

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

backend/src/main/java/org/raddatz/familienarchiv/model/PersonNameAliasType.java

@@ -4,5 +4,6 @@ public enum PersonNameAliasType {
     BIRTH,
     WIDOWED,
     DIVORCED,
+    MAIDEN_NAME,
     OTHER
 }

backend/src/main/java/org/raddatz/familienarchiv/model/PersonType.java

@@ -0,0 +1,9 @@
+package org.raddatz.familienarchiv.model;
+
+public enum PersonType {
+    PERSON,
+    INSTITUTION,
+    GROUP,
+    UNKNOWN,
+    SKIP
+}