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/lib/link-suggestions.ts

@@ -0,0 +1,72 @@
+import { prisma } from '@/lib/prisma'
+
+export interface LinkSuggestion {
+  term: string
+  noteId: string
+  noteTitle: string
+}
+
+/**
+ * Find potential wiki-link suggestions in content.
+ * Returns notes whose titles appear in the content and could be converted to [[links]].
+ */
+export async function findLinkSuggestions(
+  content: string,
+  currentNoteId?: string
+): Promise<LinkSuggestion[]> {
+  if (!content.trim() || content.length < 10) {
+    return []
+  }
+
+  // Get all note titles except current note
+  const allNotes = await prisma.note.findMany({
+    where: currentNoteId ? { id: { not: currentNoteId } } : undefined,
+    select: { id: true, title: true },
+  })
+
+  if (allNotes.length === 0) {
+    return []
+  }
+
+  // Find titles that appear in content (case-insensitive)
+  const suggestions: LinkSuggestion[] = []
+  const contentLower = content.toLowerCase()
+
+  for (const note of allNotes) {
+    const titleLower = note.title.toLowerCase()
+    // Check if title appears as a whole word in content
+    const regex = new RegExp(`\\b${escapeRegex(titleLower)}\\b`, 'i')
+    if (regex.test(content)) {
+      suggestions.push({
+        term: note.title,
+        noteId: note.id,
+        noteTitle: note.title,
+      })
+    }
+  }
+
+  // Sort by title length (longer titles first - more specific matches)
+  return suggestions.sort((a, b) => b.noteTitle.length - a.noteTitle.length)
+}
+
+/**
+ * Replace terms in content with wiki-links
+ */
+export function applyWikiLinks(
+  content: string,
+  replacements: { term: string; noteId: string }[]
+): string {
+  let result = content
+
+  for (const { term, noteId } of replacements) {
+    // Replace all occurrences of the term (case-insensitive, whole word only)
+    const regex = new RegExp(`\\b(${escapeRegex(term)})\\b`, 'gi')
+    result = result.replace(regex, `[[${term}]]`)
+  }
+
+  return result
+}
+
+function escapeRegex(str: string): string {
+  return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+}