fix(rss): emit accurate image enclosure MIME types

[Daisuke134]

Problem

The live RSS feed labels every image enclosure as image/webp, including JPEG responses such as https://picsum.photos/seed/qrcms33zi8/1000/1250 (which resolves with content-type: image/jpeg). RSS 2.0 requires the enclosure type to be a standard MIME type: https://www.rssboard.org/rss-specification#ltenclosuregtSubelementOfLtitemgt

Fix

• infer MIME types from known image URL extensions (jpg/jpeg, png, gif, webp, avif, svg)
• omit the optional enclosure when a remote URL has no trustworthy type instead of asserting WebP
• keep the existing image preview in the RSS description
• add regression coverage for JPEG, WebP with query parameters, and extensionless URLs

Verification

• pnpm test — 61/61 PASS
• pnpm typecheck — PASS
• pnpm lint — PASS with 5 pre-existing warnings
• pnpm build — PASS (43/43 static pages)