ThePalaceProject / web-patron / 25005389964

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Trigger configuration load on startup (PP-4124) (#126)

**Note:** This PR is stacked on #125, which is, in turn, stacked on
#124, and should not be merged until those have landed.

## Description

Trigger configuration load success or failure immediately upon startup:
- Uses the Instrumentation API to load and validate the app
configuration at server startup, rather than on the first request.
- If config loading fails — missing `CONFIG_FILE`, unreachable URL,
invalid YAML — the error message is printed to stderr and the process
fails fast, exiting with an error before serving any requests.

## Motivation and Context

This change makes misconfiguration immediately visible when deploying or
starting the server locally. Previously, missing or invalid
configuration would not be detected until the first request hit a
handler that called `getAppConfig()`, resulting in a silent failure or a
confusing runtime error rather than a clear startup message.

[Jira PP-4124]

## How Has This Been Tested?

- Manual testing in local dev environment, starting the server in the
following configurations with valid, invalid, and missing
file/unreachable URL:
  - without config (`CONFIG_FILE= npm run start`)
  - with local config file (`CONFIG_FILE=<filename> ...`)
  - with remote config file (`CONFIG_FILE=<url> ...`)
- All checks pass locally.
- [CI
checks](https://github.com/ThePalaceProject/web-patron/actions/runs/24896724703)
pass.

## Checklist:

- [x] I have updated the documentation accordingly.
- [x] All new and existing tests passed.

177 of 201 branches covered (88.06%)

Branch coverage included in aggregate %.

6 of 6 new or added lines in 1 file covered. (100.0%)

289 of 326 relevant lines covered (88.65%)

18.39 hits per line