feat: MVP-3 Sprint 4 - Co-usage, metrics, centrality, creation source, feature flags

- Add NoteCoUsage model and co-usage tracking when viewing notes
- Add creationSource field to notes (form/quick/import)
- Add dashboard metrics API (/api/metrics)
- Add centrality calculation (/api/centrality)
- Add feature flags system for toggling features
- Add multiline QuickAdd with smart paste type detection
- Add internal link suggestions while editing notes
- Add type inference for automatic note type detection
- Add comprehensive tests for type-inference and link-suggestions

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

prisma/schema.prisma

@@ -8,18 +8,21 @@ datasource db {
 }
 
 model Note {
-  id         String    @id @default(cuid())
-  title      String
-  content    String
-  type       String    @default("note")
-  isFavorite Boolean   @default(false)
-  isPinned   Boolean   @default(false)
-  createdAt  DateTime  @default(now())
-  updatedAt  DateTime  @updatedAt
-  tags       NoteTag[]
-  backlinks  Backlink[] @relation("BacklinkTarget")
-  outbound   Backlink[] @relation("BacklinkSource")
-  usageEvents NoteUsage[]
+  id            String    @id @default(cuid())
+  title         String
+  content       String
+  type          String    @default("note")
+  isFavorite    Boolean   @default(false)
+  isPinned      Boolean   @default(false)
+  createdAt     DateTime  @default(now())
+  updatedAt     DateTime  @updatedAt
+  creationSource String  @default("form") // 'form' | 'quick' | 'import'
+  tags          NoteTag[]
+  backlinks     Backlink[] @relation("BacklinkTarget")
+  outbound      Backlink[] @relation("BacklinkSource")
+  usageEvents   NoteUsage[]
+  coUsageFrom  NoteCoUsage[] @relation("CoUsageFrom")
+  coUsageTo    NoteCoUsage[] @relation("CoUsageTo")
 }
 
 model Tag {
@@ -60,3 +63,18 @@ model NoteUsage {
   @@index([noteId, createdAt])
   @@index([eventType, createdAt])
 }
+
+model NoteCoUsage {
+  id           String   @id @default(cuid())
+  fromNoteId   String
+  fromNote     Note     @relation("CoUsageFrom", fields: [fromNoteId], references: [id], onDelete: Cascade)
+  toNoteId     String
+  toNote       Note     @relation("CoUsageTo", fields: [toNoteId], references: [id], onDelete: Cascade)
+  weight       Int      @default(1) // times viewed together
+  createdAt    DateTime @default(now())
+  updatedAt    DateTime @updatedAt
+
+  @@unique([fromNoteId, toNoteId])
+  @@index([fromNoteId])
+  @@index([toNoteId])
+}