What kind of change does this PR introduce?

🚀 Feature

Why was this change needed?

The backend previously used a dynamic port (process.env.PORT || 3000). This caused unpredictable behavior in deployment environments where a PORT value was injected, breaking assumptions in Docker Compose and other services:

• The load balancer failed to start because it expected the backend on port 3000
• The frontend and other services couldn’t connect to the backend
• The app appeared broken despite services running

By switching to a static port (3000), the backend is consistent with:

• The documented Docker Compose setup
• Internal proxy configurations
• Expectations from other apps and services

This ensures predictable network topology and smoother deployment.

Other information

• This change was discussed in issue [#340](Create a railway.app deployment using docker #340 ) and raised again in [#871](Use static port 3000 for backend #871 ).
• The implementation is minimal: replaced const port = process.env.PORT || 3000; with const port = 3000;.
• No other functionality or dependencies were touched.

Checklist

• I have read the [CONTRIBUTING](https://github.com/gitroomhq/postiz-app/blob/main/CONTRIBUTING.md) guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just one issue and does not introduce unrelated changes.