Feature Image support for Pages

[grexe]

Summary

Pages currently support rich text body content but have no built-in featureImage field,
unlike Posts. I would like to propose a PR that adds a featureImage field to the built-in pages collection and
adds sidebar layout support in the blog/starter templates (image left or right of content).

Motivation

Pages like "About", "Contact", or "Projects" often benefit from a contextual image
displayed alongside the text — similar to how blog posts already show a feature image.
WordPress has supported this via _thumbnail_id on all post types for years.

Proposed API / UX

1. Schema: Add featureImage (type: image, optional) to the built-in pages
   collection — same as posts.
2. Template: Add an optional imagePosition field ("left" | "right", default
   "right") to control sidebar placement.
3. Rendering: When featureImage is set, wrap content in a two-column layout:
   • image column (sidebar, ~40%)
   • body column (~60%)
     Falls back gracefully to full-width if no image is set.

Affected areas

• packages/core/src/schema/built-in/pages.ts — add fields
• packages/core/src/db/migrations/ — migration for existing installs
• templates/blog/src/pages/pages/[slug].astro — render sidebar layout
• templates/starter/src/pages/pages/[slug].astro — same
• demos/ seed data — add example pages with images

Non-goals

• No change to Posts (already have featureImage)
• No new admin UI code needed (field type image already renders a media picker)
• No breaking change — field is optional, existing pages unaffected