feat: MVP-2 completion - search, quick add, backlinks, guided forms

## Search & Retrieval
- Improved search ranking with scoring (title match, favorites, recency)
- Highlight matches with excerpt extraction
- Fuzzy search with string-similarity
- Unified noteQuery function

## Quick Capture
- Quick Add API (POST /api/notes/quick) with type prefixes
- Quick add parser with tag extraction
- Global Quick Add UI (Ctrl+N shortcut)
- Tag autocomplete in forms

## Note Relations
- Automatic backlinks with sync on create/update/delete
- Backlinks API (GET /api/notes/[id]/backlinks)
- Related notes with scoring and reasons

## Guided Forms
- Type-specific form fields (command, snippet, decision, recipe, procedure, inventory)
- Serialization to/from markdown
- Tag suggestions based on content (GET /api/tags/suggest)

## UX by Type
- Command: Copy button for code blocks
- Snippet: Syntax highlighting with react-syntax-highlighter
- Procedure: Interactive checkboxes

## Quality
- Standardized error handling across all APIs
- Integration tests (28 tests passing)
- Unit tests for search, tags, quick-add

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/backlinks.ts

@@ -0,0 +1,122 @@
+import { prisma } from '@/lib/prisma'
+
+const BACKLINK_REGEX = /\[\[([^\]]+)\]\]/g
+
+export function parseBacklinks(content: string): string[] {
+  const matches = content.matchAll(BACKLINK_REGEX)
+  const titles = new Set<string>()
+
+  for (const match of matches) {
+    const title = match[1].trim()
+    if (title) {
+      titles.add(title)
+    }
+  }
+
+  return Array.from(titles)
+}
+
+export async function syncBacklinks(noteId: string, content: string): Promise<void> {
+  const linkedTitles = parseBacklinks(content)
+
+  await prisma.backlink.deleteMany({
+    where: { sourceNoteId: noteId },
+  })
+
+  if (linkedTitles.length === 0) return
+
+  const targetNotes = await prisma.note.findMany({
+    where: {
+      title: { in: linkedTitles },
+    },
+    select: { id: true, title: true },
+  })
+
+  const titleToId = new Map(targetNotes.map((n) => [n.title.toLowerCase(), n.id]))
+
+  const backlinksToCreate: { sourceNoteId: string; targetNoteId: string }[] = []
+
+  for (const title of linkedTitles) {
+    const targetNoteId = titleToId.get(title.toLowerCase())
+    if (targetNoteId && targetNoteId !== noteId) {
+      backlinksToCreate.push({
+        sourceNoteId: noteId,
+        targetNoteId,
+      })
+    }
+  }
+
+  if (backlinksToCreate.length > 0) {
+    await prisma.backlink.createMany({
+      data: backlinksToCreate,
+    })
+  }
+}
+
+export interface BacklinkWithNote {
+  id: string
+  sourceNoteId: string
+  targetNoteId: string
+  createdAt: string
+  sourceNote: {
+    id: string
+    title: string
+    type: string
+  }
+}
+
+export async function getBacklinksForNote(noteId: string): Promise<BacklinkWithNote[]> {
+  const backlinks = await prisma.backlink.findMany({
+    where: { targetNoteId: noteId },
+    include: {
+      sourceNote: {
+        select: {
+          id: true,
+          title: true,
+          type: true,
+        },
+      },
+    },
+    orderBy: { createdAt: 'desc' },
+  })
+
+  return backlinks.map((bl) => ({
+    id: bl.id,
+    sourceNoteId: bl.sourceNoteId,
+    targetNoteId: bl.targetNoteId,
+    createdAt: bl.createdAt.toISOString(),
+    sourceNote: {
+      id: bl.sourceNote.id,
+      title: bl.sourceNote.title,
+      type: bl.sourceNote.type,
+    },
+  }))
+}
+
+export async function getOutgoingLinksForNote(noteId: string): Promise<BacklinkWithNote[]> {
+  const backlinks = await prisma.backlink.findMany({
+    where: { sourceNoteId: noteId },
+    include: {
+      targetNote: {
+        select: {
+          id: true,
+          title: true,
+          type: true,
+        },
+      },
+    },
+    orderBy: { createdAt: 'desc' },
+  })
+
+  return backlinks.map((bl) => ({
+    id: bl.id,
+    sourceNoteId: bl.sourceNoteId,
+    targetNoteId: bl.targetNoteId,
+    createdAt: bl.createdAt.toISOString(),
+    sourceNote: {
+      id: bl.targetNote.id,
+      title: bl.targetNote.title,
+      type: bl.targetNote.type,
+    },
+  }))
+}