feat(#221): add V39 migration for tag hierarchy and colors

Adds parent_id FK (ON DELETE SET NULL), self-reference check constraint,
parent_id index, and nullable color column to the tag table.

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

backend/src/test/java/org/raddatz/familienarchiv/repository/MigrationIntegrationTest.java

@@ -168,8 +168,63 @@ class MigrationIntegrationTest {
         assertThat(rows).isEqualTo(1);
     }
 
+    // ─── V39: tag hierarchy — parent_id FK + self-reference check + color ──────
+
+    @Test
+    void v39_parentId_allowsNull() {
+        UUID tagId = createTag("TagWithoutParent");
+
+        Integer count = jdbc.queryForObject(
+                "SELECT COUNT(*) FROM tag WHERE id = ? AND parent_id IS NULL", Integer.class, tagId);
+        assertThat(count).isEqualTo(1);
+    }
+
+    @Test
+    void v39_selfReferenceCheck_rejectsSelfAsParent() {
+        UUID tagId = createTag("SelfRef");
+
+        assertThatThrownBy(() ->
+                jdbc.update("UPDATE tag SET parent_id = id WHERE id = ?", tagId)
+        ).isInstanceOf(DataIntegrityViolationException.class);
+    }
+
+    @Test
+    void v39_parentId_acceptsValidParent() {
+        UUID parent = createTag("Parent");
+        UUID child = createTag("Child");
+
+        int rows = jdbc.update("UPDATE tag SET parent_id = ? WHERE id = ?", parent, child);
+        assertThat(rows).isEqualTo(1);
+    }
+
+    @Test
+    void v39_color_allowsNull() {
+        UUID tagId = createTag("ColorlessTag");
+
+        Integer count = jdbc.queryForObject(
+                "SELECT COUNT(*) FROM tag WHERE id = ? AND color IS NULL", Integer.class, tagId);
+        assertThat(count).isEqualTo(1);
+    }
+
+    @Test
+    void v39_color_storesTokenName() {
+        UUID tagId = createTag("ColoredTag");
+
+        int rows = jdbc.update("UPDATE tag SET color = 'sage' WHERE id = ?", tagId);
+        String stored = jdbc.queryForObject("SELECT color FROM tag WHERE id = ?", String.class, tagId);
+
+        assertThat(rows).isEqualTo(1);
+        assertThat(stored).isEqualTo("sage");
+    }
+
     // ─── helpers ─────────────────────────────────────────────────────────────
 
+    private UUID createTag(String name) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("INSERT INTO tag (id, name) VALUES (?, ?)", id, name);
+        return id;
+    }
+
     private UUID createDocument() {
         Document doc = documentRepository.save(Document.builder()
                 .title("Testdokument")