feat: add dotenv support for environment configuration

[amitb0ra]

Summary

Add dotenv support to load static/secret configuration from .env files, as discussed in #128.

This is a standalone env support PR — no test code or other refactors are mixed in.

What changed

Static config → .env (via dotenv)

Secrets and environment-specific settings are now loaded from a .env file:

• AUTH_TOKEN — GitHub personal access token
• ORGANIZATION — GitHub org name
• ORGANIZATION_HOMEPAGE — org homepage URL
• ORGANIZATION_GITHUB_URL — org GitHub URL
• ADMIN_PASSWORD — admin panel password
• SERVER_PORT — backend API port (default: 62050)

Dynamic config → config.json (unchanged)

Runtime values that the admin panel modifies stay in config.json:

• delay, contributors, startDate, includedRepositories

Test isolation support

• Path overrides: CONFIG_PATH, DATA_PATH, LOG_PATH, ADMINDATA_PATH, DATA_BASE_PATH, CONFIG_BACKUP_PATH — tests can point these at fixture files
• LEADERBOARD_SKIP_REFRESH=1 — disables the refresh.js child process spawn so tests can start the server without hitting the GitHub API
• module.exports = { server } — allows tests to import the server programmatically

New files

• src/server/.env.example — template with all supported env vars

Updated files

• src/server/app.js — dotenv loading, env vars for static config, path overrides, skip-refresh guard, server export
• src/server/refresh.js — dotenv loading, env vars for paths and organization
• src/server/util/API.js — uses process.env.AUTH_TOKEN directly, startDate passed as parameter
• src/server/config-example.json — stripped of static fields (only dynamic values remain)
• src/server/package.json — added dotenv dependency
• .gitignore — added .env
• AGENTS.md, README.md, GEMINI.md — updated setup instructions and config reference

Setup (after this PR)

cp src/server/config-example.json src/server/config.json
cp src/server/.env.example src/server/.env
# Edit .env with your GitHub token, org, admin password
# Edit config.json with your contributors list (ships with defaults)

Related PRs

• test: add stable leaderboard regression coverage #128 — Sing-Li requested standalone dotenv support PR
• test: add e2e test suite #127 — Test suite (depends on this PR for env-based test isolation)
• refactor: env-var overrides, error handling, and cleanup for testability #130 — Server refactoring (partially superseded by this PR's env changes)

[Sing-Li]

Please remove anything that is not related to dotenv support. We don't need it on this PR.

[Sing-Li]

LGTM