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>

prisma/schema.prisma

@@ -17,6 +17,8 @@ model Note {
   createdAt  DateTime  @default(now())
   updatedAt  DateTime  @updatedAt
   tags       NoteTag[]
+  backlinks  Backlink[] @relation("BacklinkTarget")
+  outbound   Backlink[] @relation("BacklinkSource")
 }
 
 model Tag {
@@ -33,3 +35,14 @@ model NoteTag {
 
   @@id([noteId, tagId])
 }
+
+model Backlink {
+  id           String   @id @default(cuid())
+  sourceNoteId String
+  targetNoteId String
+  sourceNote   Note   @relation("BacklinkSource", fields: [sourceNoteId], references: [id], onDelete: Cascade)
+  targetNote   Note   @relation("BacklinkTarget", fields: [targetNoteId], references: [id], onDelete: Cascade)
+  createdAt    DateTime @default(now())
+
+  @@unique([sourceNoteId, targetNoteId])
+}