fix: agregar DATABASE_URL al datasource de Prisma y CMD no-fatal para migrate

- schema.prisma: url = env("DATABASE_URL") era requerido por prisma migrate deploy
- Dockerfile: cambiar && por ; para que el servidor arranque aunque migrate falle

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

backend/Dockerfile

@@ -13,4 +13,4 @@ COPY --from=builder /app/node_modules ./node_modules
 COPY --from=builder /app/package.json ./
 COPY --from=builder /app/prisma ./prisma
 EXPOSE 3001
-CMD ["sh", "-c", "npx prisma migrate deploy && node dist/main.js"]
+CMD ["sh", "-c", "npx prisma migrate deploy; node dist/main.js"]

backend/prisma/schema.prisma

@@ -4,6 +4,7 @@ generator client {
 
 datasource db {
   provider = "postgresql"
+  url      = env("DATABASE_URL")
 }
 
 model chats {