fix: restore DATABASE_URL in schema.prisma for Prisma 7 runtime client

Prisma 7's prisma.config.ts only configures the CLI, not the runtime
PrismaClient. Without url in the datasource block, the generated client
defaults to engineType "client" (WASM) which requires an adapter,
causing next-auth adapter errors on OAuth callback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -8,7 +8,7 @@ generator client {
 
 datasource db {
   provider = "sqlite"
-  // url is managed in prisma.config.ts for Prisma 7+
+  url      = env("DATABASE_URL")
 }
 
 model Account {