Add metric/status page and update workflow

[prettyleaf]

This pull request introduces a status page to the project, providing real-time metrics, recent requests, and configuration visibility, with both public and admin-only endpoints. It also adds a dedicated development Docker image build workflow and updates documentation in both English and Russian to describe these new features. The most important changes are grouped below.

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Status Page Feature:

• Adds a self-contained status page with live counters, recent requests, per-minute chart, and Prometheus metrics, available at a configurable path (GHP_STATUS_PATH) and on the admin listener. The page is protected by token authentication by default, or can be left unguarded for use behind an external auth proxy. (cmd/gh-proxy/main.go, .env.example, README.md, README.ru.md) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
• Updates the Makefile to print the status page URL when running locally.

Build and Deployment Improvements:

• Introduces a new GitHub Actions workflow (build-dev.yml) to automatically build and publish a :dev Docker image for every push to the dev branch, including commit-based tags and Telegram notifications if configured.
• Documents the new image tags and how to use them in both English and Russian READMEs. [1] [2]

Documentation Updates:

• Enhances both README.md and README.ru.md to describe the status page, its endpoints, authentication options, and usage instructions. [1] [2] [3] [4] [5] [6] [7] [8]

Minor Fixes:

• Removes unnecessary helper functions and cleans up logging in main.go.
• Simplifies docker-compose.yml by removing a redundant comment.