feat: Add PostgreSQL database layer for persistent storage

[Amarjeet325]

closes #16

• Install pg and knex for database interactions and migrations.
• Create docker-compose.yml defining postgres and redis services for local development.
• Add DATABASE_URL configuration to .env.example and src/config.js.
• Implement src/db/index.js to establish connection pooling (max: 10) and export typed query helpers.
• Add initial Knex migration script for airdrops, recipients, contract_events, and indexer_state tables.
• Add migrate and migrate:rollback npm scripts to package.json.
• Fix missing comma syntax error in package.json dependencies.

[prodbycorne]

fix branch conflicts

[prodbycorne]

fix branch conflicts and let CI pass