feat(user): add deterministic avatar color to AppUser

Adds color field assigned from an 8-colour palette keyed on the user's UUID
hash (Math.abs(id.hashCode()) % 8). Fires via @PrePersist/@PreUpdate/@PostLoad
so both new and existing users get the correct colour at runtime.

V47 migration adds the column and fixes the V46 REVOKE bug that hardcoded
role name 'app_user' instead of CURRENT_USER.

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

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

@@ -19,6 +19,10 @@ import java.util.HashSet;
 import java.util.Set;
 import java.util.UUID;
 
+import jakarta.persistence.PostLoad;
+import jakarta.persistence.PrePersist;
+import jakarta.persistence.PreUpdate;
+
 @Entity
 @Table(name = "users")
 @Data
@@ -74,6 +78,28 @@ public class AppUser {
     @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private LocalDateTime createdAt;
 
+    @Column(nullable = false)
+    @Builder.Default
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private String color = "";
+
+    private static final String[] PALETTE = {
+        "#7a4f9a", "#5a8a6a", "#3060b0", "#a0522d", "#c0446e", "#c17a00", "#0e7490", "#1d4ed8"
+    };
+
+    public static String computeColor(UUID id) {
+        return PALETTE[Math.abs(id.hashCode()) % PALETTE.length];
+    }
+
+    @PrePersist
+    @PreUpdate
+    @PostLoad
+    void deriveColor() {
+        if (id != null && (color == null || color.isEmpty())) {
+            this.color = computeColor(id);
+        }
+    }
+
     public boolean hasPermission(String permission) {
         if (groups == null || groups.isEmpty()) {
             return false;

backend/src/main/resources/db/migration/V47__add_user_color.sql

@@ -0,0 +1,8 @@
+-- Add deterministic avatar color to app_users.
+-- Assigned at application layer (AppUser.java) from a fixed 8-colour palette.
+-- Also corrects V46's REVOKE which hardcoded 'app_user' instead of CURRENT_USER.
+
+ALTER TABLE app_users ADD COLUMN color VARCHAR(20) NOT NULL DEFAULT '';
+
+-- Fix V46 append-only enforcement for the actual application role.
+REVOKE UPDATE, DELETE ON audit_log FROM CURRENT_USER;