Remove ineffective PORT env from Dockerfile

[railway-app]

Problem

The runtime stage of backend/Dockerfile contained ENV PORT=8000 with comments claiming it prevents Railway from injecting PORT=8080. This is incorrect — Railway's runtime environment variable injection always overrides Dockerfile ENV directives, so the line had no effect and was actively misleading.

Solution

Removed ENV PORT=8000 and its two associated comment lines from the runtime stage. The actual fix — setting PORT=8000 as a Railway service variable — is already applied via config, so this change is purely a dead-code cleanup that eliminates future confusion.

Changes

• Modified backend/Dockerfile

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