chore(db): migrate from SQLite to PostgreSQL

- Change Prisma provider from sqlite to postgresql
- Replace better-sqlite3 adapter with @prisma/adapter-pg
- Update prisma.config.ts and lib/prisma.ts for PostgreSQL
- Add DATABASE_URL to .env.local
- Regenerate Prisma client for PostgreSQL
- Recreate migrations for PostgreSQL database

lib/generated/prisma/models/User.ts

@@ -1264,6 +1264,7 @@ export type UserCreateManyArgs<ExtArgs extends runtime.Types.Extensions.Internal
    * The data used to create many Users.
    */
   data: Prisma.UserCreateManyInput | Prisma.UserCreateManyInput[]
+  skipDuplicates?: boolean
 }
 
 /**
@@ -1282,6 +1283,7 @@ export type UserCreateManyAndReturnArgs<ExtArgs extends runtime.Types.Extensions
    * The data used to create many Users.
    */
   data: Prisma.UserCreateManyInput | Prisma.UserCreateManyInput[]
+  skipDuplicates?: boolean
 }
 
 /**