Refactor: Consolidate documentation into README and enhance production setup

[ak4code]

Summary

This PR consolidates scattered styleguide documentation into a comprehensive README, enhances production-readiness with proper security settings and health checks, and improves the overall developer experience with better tooling integration.

Key Changes

Documentation & README

• Removed separate styleguide files (services_and_selectors.md, api_view.md, docstring.md) that were fragmented and hard to maintain
• Rewrote README.md as a comprehensive project guide with:
  • Visual header with badges (Python, Django, DRF, PostgreSQL, Redis, Docker, uv)
  • Feature matrix highlighting key capabilities
  • Quick start instructions (Docker and local setup)
  • API versioning explanation with examples
  • Architecture diagram (mermaid flowchart)
  • Project structure overview
  • Development workflow guide

Production & Security

• Added core/health.py with /health/ endpoint that checks database and Redis connectivity (returns 503 if unhealthy) — ready for Kubernetes probes
• Enhanced config/settings/prod.py with:
  • HSTS headers (30 days by default)
  • Secure cookies (HTTPS-only, HttpOnly)
  • SSL redirect behind proxy
  • Content security headers (X-Frame-Options, REFERRER-POLICY, etc.)
  • Manifest static files storage with WhiteNoise
• Updated config/settings/base.py:
  • Made .env optional (graceful fallback if missing)
  • Added CSRF_TRUSTED_ORIGINS support
  • Integrated drf-spectacular for OpenAPI/Swagger
  • Configured AcceptHeaderVersioning (version in Accept header, not URL)
  • Added django-filters and pagination defaults
  • Added WhiteNoise middleware for static file serving
  • Connection pooling and health checks for PostgreSQL

API & Versioning

• Implemented AcceptHeaderVersioning — clients specify version via Accept: application/json; version=v1 header instead of URL paths
• Updated test URLs from /api/v1/users/... to /api/users/... (version now in header)
• Added test case for version header validation and unsupported version handling
• Enhanced views with @extend_schema decorators for OpenAPI documentation

Docker & Deployment

• Added PostgreSQL service to docker-compose.yml with health checks and persistent volumes
• Updated entrypoint.sh to wait for both PostgreSQL and Redis before migrations
• Improved Dockerfile Python version handling (simplified to 3.13 without patch version)
• Enhanced Makefile with comprehensive targets: install, migrate, test, lint, format, up, down, logs

CI/CD

• Added .github/workflows/ci.yml with:
  • Ruff linting and formatting checks
  • Bandit security scanning
  • Pytest test suite execution

Dependencies

• Updated core packages: Django 6.0.6, DRF 3.17.1, Redis 8.0.0
• Added production essentials: gunicorn, psycopg[binary,pool], whitenoise, drf-spectacular
• Updated dev tools: ruff 0.15.17, bandit 1.9.4, pytest-django 4.12.0, added pytest-env

Code Quality

• Fixed formatting issues across test files (quote consistency, blank lines)
• Improved migration formatting for readability
• Added docstrings to admin and app config classes
• Cleaned up unused imports and reorganized imports per ruff standards

Notable Implementation Details

1. API Versioning: The AcceptHeaderVersioning approach keeps URLs clean and stable while allowing multiple API versions to coexist. Default version is v1; unsupported versions return 406.

2. Health Check: The /health/ endpoint is non-authenticated and checks both database and