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/note-connections.tsx

@@ -0,0 +1,142 @@
+'use client'
+
+import Link from 'next/link'
+import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
+import { Badge } from '@/components/ui/badge'
+import { ArrowRight, Link2, RefreshCw, ExternalLink } from 'lucide-react'
+
+interface BacklinkInfo {
+  id: string
+  sourceNoteId: string
+  targetNoteId: string
+  sourceNote: {
+    id: string
+    title: string
+    type: string
+  }
+}
+
+interface RelatedNote {
+  id: string
+  title: string
+  type: string
+  tags: string[]
+  score: number
+  reason: string
+}
+
+interface NoteConnectionsProps {
+  noteId: string
+  backlinks: BacklinkInfo[]
+  outgoingLinks: BacklinkInfo[]
+  relatedNotes: RelatedNote[]
+}
+
+function ConnectionGroup({
+  title,
+  icon: Icon,
+  notes,
+  emptyMessage,
+}: {
+  title: string
+  icon: React.ComponentType<{ className?: string }>
+  notes: { id: string; title: string; type: string }[]
+  emptyMessage: string
+}) {
+  if (notes.length === 0) {
+    return (
+      <div className="space-y-2">
+        <h4 className="text-sm font-medium flex items-center gap-2 text-muted-foreground">
+          <Icon className="h-4 w-4" />
+          {title}
+        </h4>
+        <p className="text-xs text-muted-foreground pl-6">{emptyMessage}</p>
+      </div>
+    )
+  }
+
+  return (
+    <div className="space-y-2">
+      <h4 className="text-sm font-medium flex items-center gap-2">
+        <Icon className="h-4 w-4" />
+        {title}
+        <Badge variant="secondary" className="ml-auto text-xs">
+          {notes.length}
+        </Badge>
+      </h4>
+      <div className="pl-6 space-y-1">
+        {notes.map((note) => (
+          <Link
+            key={note.id}
+            href={`/notes/${note.id}`}
+            className="block text-sm text-foreground hover:text-primary transition-colors"
+          >
+            {note.title}
+          </Link>
+        ))}
+      </div>
+    </div>
+  )
+}
+
+export function NoteConnections({
+  noteId,
+  backlinks,
+  outgoingLinks,
+  relatedNotes,
+}: NoteConnectionsProps) {
+  const hasAnyConnections =
+    backlinks.length > 0 || outgoingLinks.length > 0 || relatedNotes.length > 0
+
+  if (!hasAnyConnections) {
+    return null
+  }
+
+  return (
+    <Card>
+      <CardHeader>
+        <CardTitle className="text-lg flex items-center gap-2">
+          <Link2 className="h-5 w-5" />
+          Conectado con
+        </CardTitle>
+      </CardHeader>
+      <CardContent className="space-y-4">
+        {/* Backlinks - notes that link TO this note */}
+        <ConnectionGroup
+          title="Enlaces entrantes"
+          icon={ExternalLink}
+          notes={backlinks.map((bl) => ({
+            id: bl.sourceNote.id,
+            title: bl.sourceNote.title,
+            type: bl.sourceNote.type,
+          }))}
+          emptyMessage="Ningún otro documento enlaza a esta nota"
+        />
+
+        {/* Outgoing links - notes this note links TO */}
+        <ConnectionGroup
+          title="Enlaces salientes"
+          icon={ArrowRight}
+          notes={outgoingLinks.map((ol) => ({
+            id: ol.sourceNote.id,
+            title: ol.sourceNote.title,
+            type: ol.sourceNote.type,
+          }))}
+          emptyMessage="Esta nota no enlaza a ningún otro documento"
+        />
+
+        {/* Related notes - by content similarity and scoring */}
+        <ConnectionGroup
+          title="Relacionadas"
+          icon={RefreshCw}
+          notes={relatedNotes.map((rn) => ({
+            id: rn.id,
+            title: rn.title,
+            type: rn.type,
+          }))}
+          emptyMessage="No hay notas relacionadas"
+        />
+      </CardContent>
+    </Card>
+  )
+}