Add RSS 2.0 and Atom 1.0 feed support
Introduces `maudit::feed` module with `RssFeed`, `RssItem`, `AtomFeed`,
and `AtomEntry` builder types. Feeds are implemented as regular routes
returning these types, which convert to `RenderResult::Raw` via `From`
impls. Adds a `/feed.xml` route to the blog example to demonstrate usage.
https://claude.ai/code/session_01Lvoa6K3XPhuijkc6DQYY5L