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>

src/app/api/export-import/route.ts

@@ -79,6 +79,7 @@ export async function POST(req: NextRequest) {
                 id: item.id,
                 createdAt,
                 updatedAt,
+                creationSource: 'import',
               },
             })
             processed++
@@ -99,6 +100,7 @@ export async function POST(req: NextRequest) {
                 ...noteData,
                 createdAt,
                 updatedAt,
+                creationSource: 'import',
               },
             })
           }