feat: support multi-approver steps with ANY/ALL policy

- ApprovalStep now has candidates + ApprovalPolicy (ANY/ALL) instead of a single assignee; ApprovalStep.single(...) kept as a convenience factory for the single-approver case
- AssigneeResolver resolves per-candidate; TaskStatus gains SKIPPED for auto-skipped sibling tasks
- DefaultOrdoEngine creates one task per candidate and advances per-step according to policy (ANY: first approval wins and skips the rest, only rejects once everyone rejects; ALL: fail-fast on first rejection, only advances once everyone approves)
- Added ApprovalTaskRepository.findByInstanceIdAndStepId and JDBC/in-memory implementations
- Added V2 migration: ordo_approval_step.policy column + ordo_step_candidate table
- Updated ApprovalStepMapper/ApprovalTaskMapper/JdbcProcessDefinitionRepository for the new schema, handling null action for SKIPPED tasks
- Migrated all call sites (tests, ordo-example, rhizome demo) to ApprovalStep.single(...) and the 3-arg AssigneeResolver
- Added ANY/ALL policy test coverage; fixed JdbcTestSupport to apply the V2 migration and strip SQL comments before splitting on ';'

ordo-spring-boot-autoconfigure/src/test/java/com/jetlumen/ordo/spring/OrdoJdbcAutoConfigurationTest.java

@@ -22,7 +22,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 class OrdoJdbcAutoConfigurationTest {
 
     private static final ProcessDefinition LEAVE_REQUEST = new ProcessDefinition("leave-request", "Leave request",
-            List.of(new ApprovalStep("manager", "Manager approval", "maria")));
+            List.of(ApprovalStep.single("manager", "Manager approval", "maria")));
 
     private final ApplicationContextRunner withDataSourceRunner = new ApplicationContextRunner()
             .withConfiguration(AutoConfigurations.of(
@@ -71,7 +71,7 @@ class OrdoJdbcAutoConfigurationTest {
                 .run(context -> {
                     assertThat(context).hasSingleBean(OrdoEngine.class);
                     assertThat(context.getBean(AssigneeResolver.class)).isNotNull();
-                    assertThat(context.getBean(AssigneeResolver.class).resolve(null, null))
+                    assertThat(context.getBean(AssigneeResolver.class).resolve(null, null, null))
                             .isEqualTo("custom-assignee");
                 });
     }
@@ -80,7 +80,7 @@ class OrdoJdbcAutoConfigurationTest {
     static class CustomAssigneeResolverConfig {
         @Bean
         AssigneeResolver ordoAssigneeResolver() {
-            return (step, ctx) -> "custom-assignee";
+            return (candidate, step, ctx) -> "custom-assignee";
         }
     }
 }