feat: initial approval workflow engine with in-memory and JDBC storage

Lightweight linear approval engine (v0.1):

- ordo-api: domain model, OrdoEngine port, repository SPI with
  conditional updates (insertIfAbsent, completeIfPending) and a
  TransactionExecutor port for atomic multi-step writes
- ordo-core: DefaultOrdoEngine running register/start/approve/reject
  inside a transaction boundary; in-memory engine and repositories
- ordo-storage-jdbc: thread-bound JDBC transactions, normalized V1
  schema migration, Jackson-based ProcessContext JSON codec
- tests: unit tests plus H2 integration tests; PostgreSQL integration
  tests run against a local instance via ordo.test.pg.* properties and
  skip when unreachable

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

.idea/.gitignore

@@ -0,0 +1,10 @@
+# Default ignored files
+/shelf/
+/workspace.xml
+# Editor-based HTTP Client requests
+/httpRequests/
+# Ignored default folder with query files
+/queries/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml