feat: embedding dimension validation + migration system

- Add /api/admin/embeddings/dimension (GET column dim, POST test model dim)
- Add /api/admin/embeddings/migrate (alter column, clear, re-index)
- Admin form warns on dimension mismatch after save, offers migrate button
- Remove hardcoded 1536 from validate endpoint and embedding service
- Add validateDimension() utility to EmbeddingService
- Fix health route: import prisma correctly, use router instead of missing registry
- i18n keys for dimension warning (EN/FR)

memento-note/app/api/admin/embeddings/dimension/route.ts

@@ -0,0 +1,59 @@
+import { NextResponse } from 'next/server'
+import { prisma } from '@/lib/prisma'
+import { auth } from '@/auth'
+import { embeddingService } from '@/lib/ai/services/embedding.service'
+
+export async function GET() {
+  try {
+    const session = await auth()
+    if (!session?.user?.id || (session.user as { role?: string }).role !== 'ADMIN') {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+    }
+
+    const colDimResult: Array<{ dim: number | null }> = await prisma.$queryRawUnsafe(
+      `SELECT a.atttypmod AS dim FROM pg_attribute a JOIN pg_class c ON a.attrelid = c.oid WHERE c.relname = 'NoteEmbedding' AND a.attname = 'embedding'`
+    )
+    const dbDimension = colDimResult[0]?.dim ?? null
+
+    const countResult: Array<{ total: bigint }> = await prisma.$queryRawUnsafe(
+      `SELECT COUNT(*)::bigint AS total FROM "NoteEmbedding"`
+    )
+    const embeddingCount = Number(countResult[0]?.total ?? 0)
+
+    return NextResponse.json({
+      success: true,
+      dbDimension,
+      embeddingCount,
+    })
+  } catch (error) {
+    console.error('[EMBEDDING_DIMENSION] Error:', error)
+    return NextResponse.json({ success: false, error: String(error) }, { status: 500 })
+  }
+}
+
+export async function POST() {
+  try {
+    const session = await auth()
+    if (!session?.user?.id || (session.user as { role?: string }).role !== 'ADMIN') {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+    }
+
+    const result = await embeddingService.generateEmbedding('dimension test')
+    const modelDimension = result.dimension
+
+    const colDimResult: Array<{ dim: number | null }> = await prisma.$queryRawUnsafe(
+      `SELECT a.atttypmod AS dim FROM pg_attribute a JOIN pg_class c ON a.attrelid = c.oid WHERE c.relname = 'NoteEmbedding' AND a.attname = 'embedding'`
+    )
+    const dbDimension = colDimResult[0]?.dim ?? null
+
+    return NextResponse.json({
+      success: true,
+      modelDimension,
+      dbDimension,
+      match: modelDimension === dbDimension,
+    })
+  } catch (error) {
+    console.error('[EMBEDDING_DIMENSION] Test error:', error)
+    return NextResponse.json({ success: false, error: String(error) }, { status: 500 })
+  }
+}