fix: support the standard PORT environment variable - #34

Addressed Issues:

Fixes #26

What this changes

The server only read RELAY_SERVER_PORT. Managed platforms — Render, Railway, Heroku, Cloud Run — inject a PORT variable and expect the process to bind to it, so ThruBox kept binding to 3000 and deploys either failed their health checks or leaned on the platform's port auto-detection.

applyEnvOverrides now reads PORT as a fallback. RELAY_SERVER_PORT stays authoritative when both are present, so nothing changes for existing deployments.

One deliberate detail: an invalid RELAY_SERVER_PORT does not fall through to PORT. It warns and keeps the default, exactly as before. Falling through would mean a typo in the explicit variable silently hands control to the implicit one, which is the kind of surprise this issue is trying to remove.

Screenshots/Recordings:

Not applicable — server-side change. Verified against a real build instead, one run per acceptance criterion, with a live request to prove the socket is actually bound:

AC1  PORT=8080 only                   -> addr=0.0.0.0:8080 | GET :8080/health = 200
AC2  RELAY_SERVER_PORT=3000 PORT=8080 -> addr=0.0.0.0:3000 | GET :3000/health = 200
AC3  neither set                      -> addr=0.0.0.0:3000 | GET :3000/health = 200

Acceptance criteria from the issue:

• PORT=8080 ./relay-server listens on 8080
• RELAY_SERVER_PORT=3000 PORT=8080 ./relay-server listens on 3000
• Neither set → still 3000
• README config table documents PORT as a fallback

Additional Notes:

This adds the first test file in the repository (internal/config/config_test.go). It covers the full precedence matrix including both invalid-value paths. The README line that said "No test files exist in the repository yet" is updated accordingly.

$ go test ./internal/config/
ok  	github.com/AOSSIE-Org/ThruBox-Server/internal/config

gofmt and go vet are clean on every file this PR touches.

Reviewing alongside #27 and #32: all three were checked against each other before opening. Every pairwise and three-way merge is clean, and the merged tree builds and passes tests in all orders tested. No merge order is required.

Out of scope, spotted while working here (each wants its own issue):

• internal/middleware/ratelimit.go is not gofmt-clean on main — the visitor struct fields are misaligned. Untouched here.
• .gitignore:34 has a bare relay entry that matches the cmd/relay/ directory, so any new file added to that package is silently ignored by git add. Existing tracked files are unaffected, which is why it has gone unnoticed.
• dangerfile.js requires a checklist item "My PR addresses a single issue" that does not exist in .github/PULL_REQUEST_TEMPLATE.md, so every PR is flagged for an unticked box nobody can tick. I have added the line manually below.

Checklist

• My PR addresses a single issue
• My code follows the project's code style and conventions
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have joined the Discord server and I will share a link to this PR with the project maintainers there
• I have read the Contributing Guidelines

⚠️ AI Notice

This PR was drafted with Claude Code, model Claude Opus 5.

• Scope of AI assistance: implementation, tests, README changes, and this description.
• Verification: every acceptance criterion was executed against a compiled binary with a live HTTP check, not asserted from reading the code. go build, go vet, gofmt and go test were run and are reported above.
• Opened as a draft for maintainer review.

Summary by CodeRabbit

• New Features

  • Server deployments can now use the standard PORT environment variable when RELAY_SERVER_PORT is not set.
  • Added deployment guidance for managed hosting platforms.

• Bug Fixes

  • Invalid port values now produce a warning while preserving the existing configuration.

• Documentation

  • Updated testing instructions with guidance on co-located tests and an example test file.