Migrate database from MySQL to PostgreSQL

[Davictory2003]

No description provided.

[Copilot]

Pull request overview

This PR updates the local/dev Docker setup to switch the application database from MySQL to PostgreSQL.

Changes:

• Swap the db service in docker-compose.yml from mysql:8.0 to postgres:15, including env vars, port comment, and volume path/name.
• Update backend database connection URLs from mysql://... to postgres://... in both compose and .env.docker.
• Replace MySQL client dependencies with PostgreSQL ones (psycopg2-binary, libpq-dev, postgresql-client) and update the container startup wait check to pg_isready.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docker-compose.yml	Switches the DB container to Postgres and updates backend DATABASE_URL + volume.
backend/requirements.txt	Replaces mysqlclient with psycopg2-binary for Postgres connectivity.
backend/Dockerfile	Installs Postgres client/libs and waits for Postgres readiness before migrations.
backend/.env.docker	Updates the documented Docker DATABASE_URL to Postgres.

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