docs: expand Docker Compose local dev section

[vanshikagupta2005]

Description

Expanded the Docker Compose section in README.md which previously only contained
a single command (docker compose up --build) with no setup guidance.

Problem

New contributors had no documentation on how to run the full local stack
(backend + frontend + PostgreSQL) using Docker Compose.

Changes made

Added a detailed local development section covering:

• Prerequisites (Docker + Docker Compose)
• Environment variable setup with database credentials
• Service table showing Frontend, Backend, and PostgreSQL URLs
• Health check verification steps
• Stop commands including volume cleanup

Expected Behavior

A contributor can follow the README alone to spin up the full stack locally
and verify it works via curl http://localhost:8000/healthz/ready

Related Issue

Fixes #462

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: docs: add Docker Compose local development section
• I have added tests for new features (Level 2 and 3 issues) — N/A (docs only)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A — documentation change only.

Test evidence

N/A — this is a documentation-only change. No code was modified,
so no pytest run is required. Existing tests remain unaffected.