feat(backend): add Sequelize database adapter

[saad-gsoft]

Summary

• Adds complete Sequelize adapter implementing the database-agnostic repository pattern from main
• SequelizeBaseRepository with full CRUD, pagination, count(), clearAll()
• UserRepository with isEmailTaken() and password field stripping
• User/Token models with UUID primary keys and Sequelize hooks for password hashing
• SQLite in-memory for fast test execution, PostgreSQL for production
• Configures PostgreSQL in Docker Compose and GitHub Actions CI
• Sets DB_ADAPTER=sequelize and DATABASE_URL in env files

Test plan

• All 185 tests pass with SQLite in-memory
• CI pipeline runs tests with SQLite
• Verify Docker Compose brings up PostgreSQL plus app together

Generated with Claude Code

[saad-gsoft]

Fixes Applied

1. DB connection at startup

connect() from the repository adapter was only called in test setup. Added to index.js before startWorkers() and listen(). Added disconnect() on shutdown/SIGTERM.

2. Documentation updated

README.md updated to reflect database-agnostic architecture:

• Replaced MongoDB-first language with adapter-pattern language (PostgreSQL default, MongoDB supported)
• Documented DB_ADAPTER and DATABASE_URL env vars
• Removed obsolete "Mongoose Plugins" section
• Updated Docker, CI, and tech stack sections

[github-actions]

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