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/test/java/org/raddatz/familienarchiv/model/AppUserTest.java

@@ -0,0 +1,38 @@
+package org.raddatz.familienarchiv.model;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class AppUserTest {
+
+    private static final List<String> EXPECTED_PALETTE = List.of(
+        "#7a4f9a", "#5a8a6a", "#3060b0", "#a0522d", "#c0446e", "#c17a00", "#0e7490", "#1d4ed8"
+    );
+
+    @Test
+    void computeColor_returnsDeterministicPaletteColor() {
+        UUID id = UUID.fromString("12345678-1234-1234-1234-123456789abc");
+        String color = AppUser.computeColor(id);
+        assertThat(EXPECTED_PALETTE).contains(color);
+        assertThat(AppUser.computeColor(id)).isEqualTo(color);
+    }
+
+    @Test
+    void computeColor_isStableAcrossCalls() {
+        UUID id = UUID.randomUUID();
+        assertThat(AppUser.computeColor(id)).isEqualTo(AppUser.computeColor(id));
+    }
+
+    @Test
+    void computeColor_variesAcrossDifferentIds() {
+        long distinct = java.util.stream.IntStream.range(0, 100)
+            .mapToObj(i -> AppUser.computeColor(UUID.randomUUID()))
+            .distinct()
+            .count();
+        assertThat(distinct).isGreaterThan(1);
+    }
+}