fix(db): add PRIMARY KEY to group_permissions; promote tbmp UNIQUE to PK

V63 deduplicates any phantom (group_id, permission) rows accumulated since
the initial schema. V64 sets NOT NULL on permission and adds pk_group_permissions.
V65 renames uq_tbmp_block_person to pk_tbmp for naming-convention consistency.
Integration tests confirm each constraint via pg_catalog.pg_constraint. Closes #469 (partial).

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

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

@@ -399,6 +399,66 @@ class MigrationIntegrationTest {
               AND dc.annotation_id IS NOT NULL
             """;
 
+    // ─── V63+V64: group_permissions dedup + primary key ──────────────────────
+
+    @Test
+    void v64_pk_group_permissions_exists() {
+        Integer count = jdbc.queryForObject(
+                """
+                SELECT COUNT(*) FROM pg_catalog.pg_constraint c
+                JOIN pg_catalog.pg_class t ON c.conrelid = t.oid
+                WHERE t.relname = 'group_permissions'
+                  AND c.conname = 'pk_group_permissions'
+                  AND c.contype = 'p'
+                """,
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+
+    @Test
+    void v64_permission_column_isNotNullable() {
+        Integer count = jdbc.queryForObject(
+                """
+                SELECT COUNT(*) FROM information_schema.columns
+                WHERE table_schema = 'public'
+                  AND table_name = 'group_permissions'
+                  AND column_name = 'permission'
+                  AND is_nullable = 'NO'
+                """,
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+
+    @Test
+    @Transactional(propagation = Propagation.NOT_SUPPORTED)
+    void v64_rejectsDuplicateGroupPermission() {
+        UUID groupId = createUserGroup("DuplicateTestGroup-" + UUID.randomUUID());
+        jdbc.update("INSERT INTO group_permissions (group_id, permission) VALUES (?, 'READ_ALL')", groupId);
+
+        assertThatThrownBy(() ->
+                jdbc.update("INSERT INTO group_permissions (group_id, permission) VALUES (?, 'READ_ALL')", groupId)
+        ).isInstanceOf(DataIntegrityViolationException.class);
+
+        jdbc.update("DELETE FROM group_permissions WHERE group_id = ?", groupId);
+        jdbc.update("DELETE FROM user_groups WHERE id = ?", groupId);
+    }
+
+    // ─── V65: tbmp UNIQUE promoted to PRIMARY KEY ─────────────────────────────
+
+    @Test
+    void v65_pk_tbmp_exists() {
+        Integer count = jdbc.queryForObject(
+                """
+                SELECT COUNT(*) FROM pg_catalog.pg_constraint c
+                JOIN pg_catalog.pg_class t ON c.conrelid = t.oid
+                WHERE t.relname = 'transcription_block_mentioned_persons'
+                  AND c.conname = 'pk_tbmp'
+                  AND c.contype = 'p'
+                """,
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+
     // ─── helpers ─────────────────────────────────────────────────────────────
 
     private UUID createPerson(String firstName, String lastName) {
@@ -482,4 +542,10 @@ class MigrationIntegrationTest {
                 """, id, recipientId, docId, commentId);
         return id;
     }
+
+    private UUID createUserGroup(String name) {
+        UUID id = UUID.randomUUID();
+        jdbc.update("INSERT INTO user_groups (id, name) VALUES (?, ?)", id, name);
+        return id;
+    }
 }