Add dog command with random picture feature

[nekeal]

Summary

Add /dog hybrid command that fetches random dog pictures from dog.ceo (https://dog.ceo) API.

Changes

• src/europython_discord/dog/dogclient.py — async HTTP client wrapping the Dog API, returns a random image URL or None on failure
• src/europython_discord/dog/config.py — DogConfig model with configurable error messages
• src/europython_discord/dog/cog.py — DogCog with /dog hybrid command; sends an embed on success or a random error message on failure
• src/europython_discord/bot.py — registers DogCog with optional DogConfig
• tests/dog/test_cog.py — unit tests for success and error paths
• Config files — prod-config.toml and test-config.toml gain a [dog] section

Notes

• Uses @commands.hybrid_command — works both as $dog prefix and /dog slash command

Resolves #341