Fix backend network errors and cleanup dev environment

[TineoC]

Description

This PR addresses the "network error" reported by users (e.g., @henry07) when running the application locally using Docker Compose. The issue was caused by a race condition where the backend attempted to connect to the database before it was fully ready (specifically during the pgvector build).

Additionally, this PR cleans up the development environment by removing deprecated Kubernetes and Devbox configuration files and documentation to simplify the onboarding process for new contributors.

Related Issue

Fixes #459

Manual Tests

1. Ran docker compose up --build on a clean environment.
2. Verified that the backend container waits for the db container's healthcheck (pg_isready) before starting.
3. Verified the JJ chatbot and feedback forms function correctly without "network error".
4. Confirmed devbox and deploy/ directories are gone and README.md is updated.

Automated Tests

• Verified Docker Compose healthcheck logic locally.

Documentation

• Updated README.md, CLAUDE.md, and docs/DATABASE_CONNECTION.md to reflect the simplified Docker Compose workflow and removed all Kubernetes/Devbox references.

Reviewers

@Sahil S

Notes

• Dockerfile.prod is retained for CI/CD compatibility, but docker-compose.prod.yml was removed as it was redundant for local development.
• The environment file typo (config/env/env.dev -> config/env/dev.env) has been corrected across all documentation.