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/health/route.ts

@@ -1,5 +1,5 @@
 import { NextResponse } from 'next/server'
-import { getPrisma } from '@/lib/prisma'
+import { prisma } from '@/lib/prisma'
 import { redis } from '@/lib/redis'
 
 export const dynamic = 'force-dynamic'
@@ -11,7 +11,6 @@ export async function GET() {
   // Database check
   try {
     const dbStart = Date.now()
-    const prisma = getPrisma()
     const [noteCount, notebookCount, userCount] = await Promise.all([
       prisma.note.count(),
       prisma.notebook.count(),
@@ -47,13 +46,15 @@ export async function GET() {
 
   // AI providers check
   try {
-    const { getAIProvider, getChatProvider } = await import('@/lib/ai/providers/registry')
-    const embeddingProvider = getAIProvider()
-    const chatProvider = getChatProvider()
+    const { getSystemConfig } = await import('@/lib/config')
+    const config = await getSystemConfig()
+    const { resolveAiRoute } = await import('@/lib/ai/router')
+    const tagsRoute = resolveAiRoute('tags', config)
+    const chatRoute = resolveAiRoute('chat', config)
     checks.ai = {
       status: 'configured',
-      embedding: { provider: embeddingProvider },
-      chat: { provider: chatProvider },
+      embedding: { provider: tagsRoute.providerType, model: tagsRoute.modelName },
+      chat: { provider: chatRoute.providerType, model: chatRoute.modelName },
     }
   } catch (e) {
     checks.ai = { status: 'unhealthy', error: e instanceof Error ? e.message : 'Unknown error' }