feat(cluster): implement cluster detection and bridge notes discovery

Add automatic note clustering using density-based algorithm (DBSCAN variant)
and bridge notes detection for connecting different thematic clusters.

Features:
- NoteCluster, ClusterMember, BridgeNote, BridgeSuggestion models
- Clustering service with pgvector cosine similarity
- Bridge notes detection (notes connecting >=2 clusters)
- AI-powered suggestions for missing cluster connections
- /insights page with React Flow visualization
- Cron endpoint for automatic recalculation

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memento-note/app/api/bridge-notes/suggestions/route.ts

@@ -0,0 +1,66 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { auth } from '@/auth'
+import { bridgeNotesService } from '@/lib/ai/services/bridge-notes.service'
+
+/**
+ * GET /api/bridge-notes/suggestions
+ * Get AI-powered bridge note suggestions for unconnected clusters.
+ */
+export async function GET(request: NextRequest) {
+  try {
+    const session = await auth()
+    if (!session?.user?.id) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+    }
+
+    const userId = session.user.id
+    const { searchParams } = new URL(request.url)
+    const includeDismissed = searchParams.get('dismissed') === 'true'
+
+    const suggestions = await bridgeNotesService.getBridgeSuggestions(userId, includeDismissed)
+
+    return NextResponse.json({
+      suggestions,
+      count: suggestions.length
+    })
+  } catch (error) {
+    console.error('Error fetching bridge suggestions:', error)
+    return NextResponse.json(
+      { error: 'Failed to fetch suggestions' },
+      { status: 500 }
+    )
+  }
+}
+
+/**
+ * POST /api/bridge-notes/suggestions/generate
+ * Generate new bridge suggestions (force regeneration).
+ */
+export async function POST(request: NextRequest) {
+  try {
+    const session = await auth()
+    if (!session?.user?.id) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+    }
+
+    const userId = session.user.id
+
+    // Generate new suggestions
+    const suggestions = await bridgeNotesService.generateBridgeSuggestions(userId)
+
+    // Save to database
+    await bridgeNotesService.saveBridgeSuggestions(userId, suggestions)
+
+    return NextResponse.json({
+      suggestions,
+      count: suggestions.length,
+      message: `Generated ${suggestions.length} bridge suggestions`
+    })
+  } catch (error) {
+    console.error('Error generating bridge suggestions:', error)
+    return NextResponse.json(
+      { error: 'Failed to generate suggestions' },
+      { status: 500 }
+    )
+  }
+}