Document local PostgreSQL service

[DataTideHH]

What changed

• add PostgreSQL and Docker Compose to the documented stack
• add commands for starting and stopping the local PostgreSQL service
• clarify that H2 remains the active application and test database
• update database notes, demonstrated capabilities and current limitations

Scope

This pull request only updates the README. It does not change the application configuration or connect Spring Boot to PostgreSQL.

Validation

• documentation checked against the current compose.yaml
• no Java, Maven or runtime behavior changed

[Copilot]

🟡 Changes recommended

The README’s PostgreSQL connection details and volume naming are slightly misleading/incomplete relative to compose.yaml.

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Pull request overview

Updates the project documentation to describe the included Docker Compose PostgreSQL service while clarifying that the Spring Boot application continues to use H2 for now.

Changes:

• Expanded the documented tech stack and “What this demonstrates” sections to include PostgreSQL + Docker Compose.
• Added local commands to start/stop the PostgreSQL service via compose.yaml.
• Reworked database notes and limitations to clearly state PostgreSQL is provisioned but not yet wired into the app.

File summaries

File	Description
README.md	Documents local PostgreSQL (Docker Compose) alongside the current H2-based runtime/testing setup.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Lite

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