feat: support per-blog User-Agent overrides for feeds

[Sowevo]

Summary

• add per-blog --user-agent on blogwatcher add (similar to --scrape-selector)
• remove global --user-agent to keep behavior simple and explicit
• scanning now uses per-blog UA only (or default UA if not configured)

Why

Different sites may require different User-Agent values. Per-blog settings are clearer and avoid confusing precedence rules.

Changes

• model/storage:
  • add user_agent field to Blog
  • add user_agent column in DB schema (with migration via ALTER TABLE ... ADD COLUMN)
• CLI:
  • blogwatcher add ... --user-agent "..." to persist per-blog UA
  • blogwatcher blogs now prints configured per-blog UA
  • removed global --user-agent flag
• scanner:
  • use only blog-specific UA when present
  • pass effective UA to feed discovery, feed fetch, and scraper fetch
• docs:
  • README examples for per-blog UA

Verification

• go test ./... passes
• blogwatcher --help no longer shows global --user-agent

[Sowevo]

Sorry for the PR churn — I moved too fast 🙇
I’ve simplified it to per-blog --user-agent only and removed the global fallback.
Thanks for the patience 🐾