Add PostgreSQL datamodel implementation

[Copilot]

Adds PostgreSQL as a third datamodel backend alongside the existing Firestore and Memory implementations.

Changes

• New module datamodel-postgres with Spring Data JPA

  • Entity classes mapping to 8 PostgreSQL tables
  • JPA repositories with query methods for pagination and lookups
  • Repository implementations for all 5 datamodel interfaces
  • Spring config activated via datamodel-postgres profile

• Documentation

  • docs/modules/datamodel/postgres.md with config reference and DDL schema
  • Updated index to list Postgres alongside other implementations

Usage

# application.properties
spring.profiles.active=datamodel-postgres
spring.datasource.url=jdbc:postgresql://localhost:5432/sac
spring.datasource.username=user
spring.datasource.password=pass
spring.jpa.hibernate.ddl-auto=update

Structure

datamodel-postgres/
├── entity/          # JPA entities (AuthorizationEntity, ClientEntity, etc.)
├── repository/      # JpaRepository interfaces + Postgres*Repository impls
├── mapper/          # MapStruct mapper for pagination DTOs
└── config/          # DatamodelPostgresConfig (@Profile("datamodel-postgres"))

Original prompt

Implement a new datamodel for this appliation. Currently it only supports Firestore and Memory. I want a Postgres backend to be implemented. Take care to analyze the Firestore implementation so you understand where and how the other modules need to be updated, the file structures used to implement a data model, and the documentation required.

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