Feat/env db provider

[AugistineCreates]

Closes #251

This PR moves the Prisma database provider selection to an environment-driven switcher. This allows switching between SQLite and PostgreSQL dynamically without modifying schema.prisma.

Summary of Changes

• Prisma Configuration Update:
  • Updated schema.prisma to use provider = env("DATABASE_PROVIDER") and shadowDatabaseUrl = env("SHADOW_DATABASE_URL").
  • Configured prisma.config.ts to dynamically intercept schema compilation and output a resolved, runtime-specific schema (schema.generated.prisma) corresponding to the configured DATABASE_PROVIDER.
• Environment Templates: Added default configurations and instructions for PostgreSQL and SQLite in .env.example.
• CI Matrix Integration: Added a matrix test execution to .github/workflows/backend-ci.yml that provisions a PostgreSQL service container and runs schema/migration checks on both sqlite and postgresql environments.
• Documentation: Authored docs/db/providers.md to guide developers on configuring connection strings, handling shadow databases, and running migrations for specific engines.

[kilodesodiq-arch]

Hi 👋 Appreciate this PR! The CI is flagging a failing check — could you take a peek at the failed job and push a fix? Happy to help if you need a hand 🙏