Feature/rest for grocery list

[JonathanPschl]

Adds read-only REST endpoints to grocery-service for retrieving stored grocery lists, backed by JPA against grocery-db, and wires them into the OpenAPI docs.

Endpoints
All routed through the api-gateway and protected by the gateway JWT filter.
GET getHistory
GET findById

Changes

• Entities: GroceryList, GroceryItem, GroceryCategory
• Repository: GroceryListRepository with findTop20ByOrderByCreatedAtDesc
• Service: maps entities to DTOs, throws 404 for unknown ids
• DTOs: keep entities out of the API surface
• OpenAPI: GroceryListController annotated so endpoints render under a "Grocery Lists" tag with
  bearerAuth + documented responses
• Config: gateway route + datasource/JPA properties for grocery-db

[timn21]

looks good