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)

.cursor/hooks/state/continual-learning.json

@@ -1,8 +1,8 @@
 {
   "version": 1,
-  "lastRunAtMs": 1779026397247,
-  "turnsSinceLastRun": 7,
-  "lastTranscriptMtimeMs": 1779026397153.5342,
-  "lastProcessedGenerationId": "2cd15cc8-e19e-4dc7-b37a-7d6d7b76ae2b",
+  "lastRunAtMs": 1779034436676,
+  "turnsSinceLastRun": 13,
+  "lastTranscriptMtimeMs": 1779034436585.3164,
+  "lastProcessedGenerationId": "fac69259-f459-4519-94aa-2b72a9453b24",
   "trialStartedAtMs": null
 }