Migrate database from SQLite to PostgreSQL

- Replace SQLite with PostgreSQL using Neon as provider
- Add environment-based database URL configuration
- Create separate test database setup with DATABASE_URL_TEST
- Reset migration history and generate fresh PostgreSQL schema
- Add comprehensive migration documentation
- Include database unit tests for connection validation

prisma/schema.prisma

@@ -3,8 +3,8 @@ generator client {
 }
 
 datasource db {
-  provider = "sqlite" // still OK for local/dev; use Postgres in prod
-  url      = "file:./dev.db"
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
 }
 
 /**