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.

db/migrations/008_connected_pages.sql

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+-- 008_connected_pages.sql — Notion-like upgrades phase 3.
+--
+-- Nested pages, resolved internal page links (backlinks), and live collection
+-- views embedded in a page. Links and embeds are derived from Markdown on
+-- every content save: the page body stays the single source of truth.
+--
+-- Access rules:
+--   * Pages remain owner-scoped. A link to a page the author does not own is
+--     ignored, so backlinks can never reveal a title across accounts.
+--   * An embed is stored only when the author owns the collection. The live
+--     view still re-checks bookmark visibility through the collection item
+--     endpoints (N-08); embedding never publishes anything.
+--
+-- Deleting:
+--   * Deleting a parent page promotes its children (SET NULL), never deletes
+--     them silently.
+--   * Deleting a page cascades its outgoing/incoming links and embeds.
+--   * Deleting a collection cascades its embeds; deleting a view clears the
+--     embed's viewId so it falls back to the collection's default view.
+
+ALTER TABLE "Page" ADD COLUMN IF NOT EXISTS "parentId" text;
+
+-- The named constraint matches schema.sql so the fresh and upgraded paths
+-- converge. The guard makes a re-run against an already-current schema a
+-- no-op (without it, schema.sql's inline FK would collide).
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1 FROM pg_constraint
+     WHERE conname = 'Page_parentId_fkey' AND conrelid = '"Page"'::regclass
+  ) THEN
+    ALTER TABLE "Page" ADD CONSTRAINT "Page_parentId_fkey"
+      FOREIGN KEY ("parentId") REFERENCES "Page"(id) ON DELETE SET NULL;
+  END IF;
+END $$;
+
+CREATE INDEX IF NOT EXISTS "Page_parentId_idx" ON "Page"("parentId");
+
+CREATE TABLE IF NOT EXISTS "PageLink" (
+  "sourcePageId" text NOT NULL REFERENCES "Page"(id) ON DELETE CASCADE,
+  "targetPageId" text NOT NULL REFERENCES "Page"(id) ON DELETE CASCADE,
+  "createdAt" timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY ("sourcePageId", "targetPageId"),
+  CONSTRAINT "PageLink_no_self" CHECK ("sourcePageId" <> "targetPageId")
+);
+CREATE INDEX IF NOT EXISTS "PageLink_targetPageId_idx" ON "PageLink"("targetPageId");
+
+CREATE TABLE IF NOT EXISTS "PageEmbed" (
+  "pageId" text NOT NULL REFERENCES "Page"(id) ON DELETE CASCADE,
+  "collectionId" text NOT NULL REFERENCES "Collection"(id) ON DELETE CASCADE,
+  "viewId" text REFERENCES "CollectionView"(id) ON DELETE SET NULL,
+  position integer NOT NULL DEFAULT 0,
+  "createdAt" timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY ("pageId", "collectionId")
+);
+CREATE INDEX IF NOT EXISTS "PageEmbed_collectionId_idx" ON "PageEmbed"("collectionId");
+CREATE INDEX IF NOT EXISTS "PageEmbed_viewId_idx" ON "PageEmbed"("viewId");