Add connected pages: hierarchy, backlinks, and live embeds

Implement Notion-like upgrade phase 3: owner-scoped nested pages with cycle rejection and child promotion on delete; Markdown-derived internal links and backlinks; live collection embeds resolved in the content-save transaction. Add migration 008, API/docs, integration coverage, and page/sidebar UI.

PLAN.md

@@ -83,7 +83,6 @@ keeping its own passwords.*
 - Collections can't be shared — they're private to their owner.
 - No page history (can't see or restore older versions).
 - No trash — deletes are permanent.
-- No nested pages or backlinks ("which pages mention this one?").
 - Calendar and list views for collections are stored but still display as
   tables.
 - Alerts only show inside the app — nothing is emailed or pushed to you.
@@ -171,8 +170,9 @@ maintenance rather than a separate milestone.
 
 - **Page history and trash** — see old versions, restore them; deleted
   things are recoverable.
-- **Nested pages and backlinks** — a real page tree, and "what links here"
-  that never shows titles you're not allowed to see.
+- **Nested pages and backlinks** — done: a real page tree, "what links here"
+  that never shows titles you're not allowed to see, and live collection
+  views embedded in pages.
 - **Collection sharing** — the big one. Explicit, shown in the UI, and
   revocable. Design the permission rules first (a project must never
   accidentally widen who can see a bookmark), then build it. Once it