feat: add Request and Job schema with migration

Adds the Prisma schema (Request/Job models, RequestStatus/JobState
enums), a Prisma client singleton at web/src/lib/db.ts, and the
init_request_job migration, applied and verified against Postgres.

Also publishes the db service's port (5432:5432) in docker-compose.yml
so host-run Prisma commands can reach localhost:5432 for local dev.

web/src/lib/db.ts

@@ -0,0 +1,7 @@
+import { PrismaClient } from "@prisma/client";
+
+const globalForPrisma = globalThis as unknown as { prisma?: PrismaClient };
+
+export const prisma = globalForPrisma.prisma ?? new PrismaClient();
+
+if (process.env.NODE_ENV !== "production") globalForPrisma.prisma = prisma;