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/(admin)/admin/settings/admin-settings-form.tsx

@@ -178,6 +178,9 @@ export function AdminSettingsForm({ config }: { config: Record<string, string> }
   const [embedFallbackProvider, setEmbedFallbackProvider] = useState<string>(config.AI_PROVIDER_EMBEDDING_FALLBACK || '')
   const [chatFallbackProvider, setChatFallbackProvider] = useState<string>(config.AI_PROVIDER_CHAT_FALLBACK || '')
 
+  const [dimensionWarning, setDimensionWarning] = useState<{ dbDimension: number; modelDimension: number } | null>(null)
+  const [isMigrating, setIsMigrating] = useState(false)
+
   // Dynamic Models State (for local providers with /api/tags or /v1/models endpoints)
   const [dynamicModels, setDynamicModels] = useState<Record<ModelPurpose, string[]>>({
     tags: [],
@@ -412,6 +415,18 @@ export function AdminSettingsForm({ config }: { config: Record<string, string> }
         toast.error(t('admin.ai.updateFailed') + ': ' + result.error)
       } else {
         toast.success(t('admin.ai.updateSuccess'))
+
+        setDimensionWarning(null)
+        try {
+          const dimRes = await fetch('/api/admin/embeddings/dimension', { method: 'POST' })
+          const dimData = await dimRes.json()
+          if (dimData.success && !dimData.match) {
+            setDimensionWarning({
+              dbDimension: dimData.dbDimension,
+              modelDimension: dimData.modelDimension,
+            })
+          }
+        } catch {}
       }
     } catch (error: any) {
       setIsSaving(false)
@@ -419,6 +434,24 @@ export function AdminSettingsForm({ config }: { config: Record<string, string> }
     }
   }
 
+  const handleMigrateEmbeddings = async () => {
+    setIsMigrating(true)
+    try {
+      const res = await fetch('/api/admin/embeddings/migrate', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: '{}' })
+      const data = await res.json()
+      if (data.success) {
+        toast.success(data.message)
+        setDimensionWarning(null)
+      } else {
+        toast.error(data.error || 'Migration failed')
+      }
+    } catch (error: any) {
+      toast.error(error.message)
+    } finally {
+      setIsMigrating(false)
+    }
+  }
+
   const handleSaveEmail = async (formData: FormData) => {
     setIsSaving(true)
     const data: Record<string, string> = { EMAIL_PROVIDER: emailProvider }
@@ -426,6 +459,8 @@ export function AdminSettingsForm({ config }: { config: Record<string, string> }
     if (emailProvider === 'resend') {
       const key = formData.get('RESEND_API_KEY') as string
       if (key) data.RESEND_API_KEY = key
+      const from = formData.get('SMTP_FROM') as string
+      if (from) data.SMTP_FROM = from
     } else {
       data.SMTP_HOST = formData.get('SMTP_HOST') as string
       data.SMTP_PORT = formData.get('SMTP_PORT') as string
@@ -772,6 +807,33 @@ export function AdminSettingsForm({ config }: { config: Record<string, string> }
               </h3>
               <p className="text-xs text-muted-foreground">{t('admin.ai.embeddingsDescription')}</p>
 
+              {dimensionWarning && (
+                <div className="rounded-lg border border-amber-500/50 bg-amber-50 dark:bg-amber-950/20 p-3 space-y-2">
+                  <p className="text-sm font-medium text-amber-800 dark:text-amber-200">
+                    {t('admin.ai.dimensionMismatch')}
+                  </p>
+                  <p className="text-xs text-amber-700 dark:text-amber-300">
+                    {t('admin.ai.dimensionMismatchDetail', {
+                      dbDimension: dimensionWarning.dbDimension,
+                      modelDimension: dimensionWarning.modelDimension,
+                    })}
+                  </p>
+                  <p className="text-xs text-amber-600 dark:text-amber-400">
+                    {t('admin.ai.dimensionMismatchAction')}
+                  </p>
+                  <Button
+                    type="button"
+                    size="sm"
+                    variant="outline"
+                    className="border-amber-500 text-amber-800 hover:bg-amber-100 dark:text-amber-200 dark:hover:bg-amber-900"
+                    disabled={isMigrating}
+                    onClick={handleMigrateEmbeddings}
+                  >
+                    {isMigrating ? t('admin.ai.migrating') : t('admin.ai.migrateButton')}
+                  </Button>
+                </div>
+              )}
+
               <div className="space-y-2">
                 <Label htmlFor="AI_PROVIDER_EMBEDDING">
                   {t('admin.ai.provider')}
@@ -972,6 +1034,15 @@ export function AdminSettingsForm({ config }: { config: Record<string, string> }
                   <Input id="RESEND_API_KEY" name="RESEND_API_KEY" type="password" defaultValue={config.RESEND_API_KEY || ''} placeholder="re_..." />
                   <p className="text-xs text-muted-foreground">{t('admin.resend.apiKeyHint')}</p>
                 </div>
+                <div className="space-y-2">
+                  <label htmlFor="SMTP_FROM" className="text-sm font-medium">{t('admin.smtp.fromEmail')}</label>
+                  <Input
+                    id="SMTP_FROM"
+                    name="SMTP_FROM"
+                    defaultValue={config.SMTP_FROM || 'noreply@memento-note.com'}
+                    placeholder="noreply@memento-note.com"
+                  />
+                </div>
                 {config.RESEND_API_KEY && (
                   <div className="flex items-center gap-2 text-xs text-green-600">
                     <span className="inline-block w-2 h-2 rounded-full bg-green-500" />