feat: Add database setup and schema for User and Preferences (#154)

[Miracle656]

Description

Closes #154

This PR introduces the MVP database setup and schema required for user management and preference tracking in the frontend/backend that operate beyond on-chain data.

Prisma with SQLite has been selected as the initial database choice. This setup requires zero extra configuration to launch locally and can easily be swapped to a production database like PostgreSQL when necessary.

Key Changes

• Prisma Setup: Installed prisma & @prisma/client (v5.22.0) to manage the schema and generate the database client.
• Initial Schema: Created prisma/schema.prisma containing models:
  • User: Handles standard user details (id, stellar_address, createdAt, updatedAt).
  • UserPreference: Tied to User containing configuration choices (currency, language, notifications_enabled).
• Database Client Singleton: Implemented lib/db.ts to instantiate a single PrismaClient globally, preventing Next.js hot-reload connection exhaustions.
• Migration & Env Config: Added dev.db initialization steps using .env mapping and configured the first database migration step init.
• Documentation: Updated README.md to walk developers through the correct local database setup when cloning or starting up the codebase.

Acceptance Criteria Validated

• DB client and migrations established in the repo.
• User and UserPreference tables created.
• README.md documents setup clearly.

How to test this PR locally

1. Checkout this branch: git checkout feature/issue-154-database-setup
2. Install the new dependencies via pnpm install or npm install.
3. Create an .env file at the project root with the following:
   ```env
   DATABASE_URL="file:./dev.db"
   ```
4. Run the Prisma migrations: npx prisma migrate dev
5. Verify the typechecker correctly identifies the Prisma client with npx tsc --noEmit.

[Baskarayelu]

LGTM!