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/schema.sql

@@ -196,6 +196,10 @@ CREATE TABLE "Page" (
   icon text,
   pinned boolean NOT NULL DEFAULT false,
   position int NOT NULL DEFAULT 0,
+  -- Connected pages (phase 3): a page may live under another page. Deleting
+  -- the parent promotes its children to the top level instead of deleting
+  -- them; cycle rejection is enforced by the API.
+  "parentId" text REFERENCES "Page"(id) ON DELETE SET NULL,
   -- Optimistic concurrency revision (review DATA-01): content saves bump it;
   -- a stale base version in PATCH is rejected with 409.
   version integer NOT NULL DEFAULT 1,
@@ -204,6 +208,7 @@ CREATE TABLE "Page" (
   "updatedAt" timestamptz NOT NULL DEFAULT now()
 );
 CREATE INDEX "Page_ownerId_position_idx" ON "Page"("ownerId", position);
+CREATE INDEX "Page_parentId_idx" ON "Page"("parentId");
 -- Index-backed substring search over the two searchable fields.
 CREATE INDEX "Page_title_trgm_idx" ON "Page" USING gin (title gin_trgm_ops);
 CREATE INDEX "Page_content_trgm_idx" ON "Page" USING gin (content gin_trgm_ops);
@@ -218,6 +223,19 @@ CREATE TABLE "PageBookmark" (
 );
 CREATE INDEX "PageBookmark_bookmarkId_idx" ON "PageBookmark"("bookmarkId");
 
+-- 2.13b PageLink: resolved internal page links, rebuilt from page content on
+-- every content save. Targets are only stored when the author owns the target
+-- page, so a backlink can never reveal an inaccessible title. Self-links are
+-- ignored by the writer and refused here as defense in depth.
+CREATE TABLE "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 "PageLink_targetPageId_idx" ON "PageLink"("targetPageId");
+
 -- 2.14 SignalSource
 CREATE TABLE "SignalSource" (
   id text PRIMARY KEY,
@@ -348,4 +366,21 @@ CREATE TABLE "CollectionView" (
   "updatedAt" timestamptz NOT NULL DEFAULT now()
 );
 CREATE INDEX "CollectionView_collectionId_idx" ON "CollectionView"("collectionId");
+
+-- 2.18 PageEmbed (Notion-like upgrades phase 3): a live collection view
+-- embedded in a page. The reference is resolved from the page's Markdown
+-- (`{{collection:<id>|view=<id>}}`) on every content save. Only collections
+-- owned by the page's author are stored; embedding displays data and never
+-- publishes it or widens bookmark access. Deleting the collection or the
+-- chosen view removes/falls back rather than leaving a dangling embed.
+CREATE TABLE "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 "PageEmbed_collectionId_idx" ON "PageEmbed"("collectionId");
+CREATE INDEX "PageEmbed_viewId_idx" ON "PageEmbed"("viewId");
 COMMIT;