feat: add MediaShare service for Telegram media hosting

[kofany]

Add self-hosted media sharing service for TeleIRC that enables sharing photos, videos, and voice messages from Telegram to IRC via web links.

Features:

• Modern responsive web player for video/audio with dark/light mode
• Image viewer with lightbox and thumbnails
• File list page with media previews and mini-players
• Automatic cleanup of expired files based on configurable retention
• Bilingual support (English/Polish)
• SQLite database for metadata storage
• API key authentication for uploads
• Docker support with included Dockerfile

Changes:

• New MediaShare microservice in internal/mediashare/
• Telegram handler integration for media uploads
• Configuration options in env.example
• Documentation in docs/user/mediashare.md
• Docker compose example with MediaShare service