[WIP] Add PostgreSQL, H2 (for testing), and environment configuration support

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Currently, the Spring Boot application is using default settings and does not have proper database configuration for development, testing, and production. We want to add support for:

1. PostgreSQL as the primary database for development and production.
2. H2 in-memory database for testing purposes.
3. .env or Spring Boot environment-based configuration to manage DB credentials and other secrets securely.

Tasks:

• Add PostgreSQL dependency in pom.xml.
• Add H2 dependency for testing.
• Configure application.properties or application.yml for multiple environments (dev, test, prod).
• Support loading environment variables from .env or OS environment.
• Ensure spring.jpa.hibernate.ddl-auto is set appropriately for each environment (update for dev, create-drop for test, validate for prod).
• Verify database connections in all environments.

Acceptance Criteria:

• The app can connect to PostgreSQL in dev/production.
• Tests run against H2 in-memory DB.
• DB credentials and sensitive info are loaded from environment variables.
• Documentation updated with steps to configure local .env and run the app.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add PostgreSQL, H2 (for testing), and environment configuration support</issue_title>
<issue_description>
Currently, the Spring Boot application is using default settings and does not have proper database configuration for development, testing, and production. We want to add support for:

1. PostgreSQL as the primary database for development and production.
2. H2 in-memory database for testing purposes.
3. .env or Spring Boot environment-based configuration to manage DB credentials and other secrets securely.

Tasks:

• Add PostgreSQL dependency in pom.xml.
• Add H2 dependency for testing.
• Configure application.properties or application.yml for multiple environments (dev, test, prod).
• Support loading environment variables from .env or OS environment.
• Ensure spring.jpa.hibernate.ddl-auto is set appropriately for each environment (update for dev, create-drop for test, validate for prod).
• Verify database connections in all environments.

Acceptance Criteria:

• The app can connect to PostgreSQL in dev/production.
• Tests run against H2 in-memory DB.
• DB credentials and sensitive info are loaded from environment variables.
• Documentation updated with steps to configure local .env and run the app.
  </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #4

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