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/clusters/route.ts

@@ -0,0 +1,124 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { auth } from '@/auth'
+import prisma from '@/lib/prisma'
+import { clusteringService } from '@/lib/ai/services/clustering.service'
+import { bridgeNotesService } from '@/lib/ai/services/bridge-notes.service'
+
+/**
+ * GET /api/clusters
+ * Get all clusters for the current user.
+ */
+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
+
+    // Check for cached results
+    const cached = await clusteringService.getCachedClusters(userId)
+    if (cached) {
+      return NextResponse.json({
+        clusters: cached,
+        cached: true,
+        totalNotes: cached.reduce((sum, c) => sum + c.noteIds.length, 0)
+      })
+    }
+
+    // No cached results, check if user has enough notes
+    const notesCount = await prisma.note.count({
+      where: { userId, trashedAt: null }
+    })
+
+    if (notesCount < 10) {
+      return NextResponse.json({
+        clusters: [],
+        message: 'Need at least 10 notes to generate clusters',
+        totalNotes: notesCount
+      })
+    }
+
+    // Trigger background recalculation
+    return NextResponse.json({
+      clusters: [],
+      message: 'Calculating clusters... Please check back later',
+      totalNotes: notesCount
+    })
+  } catch (error) {
+    console.error('Error fetching clusters:', error)
+    return NextResponse.json(
+      { error: 'Failed to fetch clusters' },
+      { status: 500 }
+    )
+  }
+}
+
+/**
+ * POST /api/clusters/recalculate
+ * Trigger a full recalculation of clusters and bridge notes.
+ */
+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
+    const body = await request.json()
+    const force = body.force === true
+
+    // Check if recalculation is needed
+    const shouldRecalc = force || await clusteringService.shouldRecalculate(userId)
+
+    if (!shouldRecalc) {
+      const cached = await clusteringService.getCachedClusters(userId)
+      if (cached) {
+        return NextResponse.json({
+          clusters: cached,
+          cached: true,
+          message: 'Using cached results (data has not changed significantly)'
+        })
+      }
+    }
+
+    // Perform clustering
+    const results = await clusteringService.clusterNotes(userId)
+
+    if (results.clusters.length === 0) {
+      return NextResponse.json({
+        clusters: [],
+        message: 'Could not generate clusters. Need more diverse notes.',
+        noiseCount: results.noiseCount
+      })
+    }
+
+    // Generate cluster names
+    for (const cluster of results.clusters) {
+      cluster.name = await clusteringService.generateClusterName(cluster.clusterId, userId)
+    }
+
+    // Save results
+    await clusteringService.saveClusteringResults(userId, results)
+
+    // Detect and save bridge notes
+    const bridgeNotes = await bridgeNotesService.detectBridgeNotes(userId)
+    await bridgeNotesService.saveBridgeNotes(userId, bridgeNotes)
+
+    return NextResponse.json({
+      clusters: results.clusters,
+      bridgeNotes: bridgeNotes.slice(0, 10), // Return top 10
+      totalNotes: results.clusters.reduce((sum, c) => sum + c.noteIds.length, 0) + results.noiseCount,
+      noiseCount: results.noiseCount,
+      message: `Generated ${results.clusters.length} clusters`
+    })
+  } catch (error) {
+    console.error('Error recalculating clusters:', error)
+    return NextResponse.json(
+      { error: 'Failed to recalculate clusters' },
+      { status: 500 }
+    )
+  }
+}