feat: MVP-3 Sprint 2 - Auto tag suggestions, note connections panel

- Add auto tag suggestions to note form with debounced API calls
- Rename related notes to "También podrías necesitar" in detail view
- Create NoteConnections component showing backlinks, outgoing links, and related notes
- Add tests for backlinks module and tags/suggest API endpoint

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

src/components/related-notes.tsx

@@ -19,7 +19,7 @@ export function RelatedNotes({ notes }: { notes: RelatedNote[] }) {
   return (
     <Card>
       <CardHeader>
-        <CardTitle className="text-lg">Notas relacionadas</CardTitle>
+        <CardTitle className="text-lg">También podrías necesitar</CardTitle>
       </CardHeader>
       <CardContent>
         <div className="space-y-3">