feat: support custom User-Agent for subscriptions

[zzzsyyy]

Background

Some providers return different node formats based on the User-Agent header.

This PR adds custom User-Agent support for subscriptions with two methods:

1. Global setting: Set subscription_user_agent in the global section, which applies to all subscriptions
2. Per-subscription setting: Set via URL fragment (e.g., https://example.com/sub#ua=xxx), which takes priority over the global setting

Checklist

• The Pull Request has been fully tested
• There's an entry in the CHANGELOGS
• There is a user-facing docs PR against https://github.com/daeuniverse/dae

Full Changelogs

• feat(subscription): support custom User-Agent for fetching subscriptions
• feat(config): add subscription_user_agent option in global section
• feat(subscription): support per-subscription User-Agent via URL fragment (#ua=xxx)

Issue Reference

Closes #929 Closes #515

Test Result