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/locales/en.json

@@ -1137,7 +1137,12 @@
       "languageDetection": "Language detection",
       "languageDetectionDesc": "Automatically detects the language of each note",
       "autoLabeling": "Auto labeling",
-      "autoLabelingDesc": "Suggests and applies labels automatically"
+      "autoLabelingDesc": "Suggests and applies labels automatically",
+      "dimensionMismatch": "Embedding dimension mismatch detected",
+      "dimensionMismatchDetail": "Database column: vector({{dbDimension}}) — Model outputs: {{modelDimension}} dimensions",
+      "dimensionMismatchAction": "Embeddings will not work until you migrate the column. This will clear all existing embeddings and re-index them.",
+      "migrateButton": "Migrate & Re-index",
+      "migrating": "Migrating..."
     },
     "resend": {
       "title": "Resend (Recommended)",