feat(config): support environment variable default values

[ndpvt-web]

Summary

Adds support for default value fallback when using environment variables in gatus configuration using the standard shell syntax ${VAR:-default}.

Closes #1523

Changes

• Added custom expandEnv() function in config/config.go that supports ${VAR:-default} syntax
• If the environment variable is set and non-empty, its value is used
• If the environment variable is unset or empty, the default value is used
• Fully backward compatible with existing $VAR and ${VAR} syntax
• Added comprehensive unit tests (15 test cases) in config/env_test.go

Example

endpoints:
  - name: My Endpoint
    url: "${API_URL:-https://api.example.com}"
    conditions:
      - "[STATUS] == ${EXPECTED_STATUS:-200}"

Test Plan

• Unit tests for default value substitution
• Tests for backward compatibility with existing syntax
• Tests for edge cases (special chars, empty defaults, nested vars)
• Manual testing with real config file

AI Disclosure

This PR was authored with assistance from an AI coding assistant (Claude). The implementation was developed transparently as part of an open-source contribution workflow. All code has been reviewed for correctness and follows the project's existing patterns.