feat: add Cache TTL Override per account + bump VERSION to 0.1.83

- Account-level cache TTL override: rewrite Anthropic cache_creation
  token classification (5m↔1h) in streaming/non-streaming responses
- New DB field cache_ttl_overridden in usage_log for billing tracking
- Migration 055_add_cache_ttl_overridden
- Frontend: CacheTTL override toggle in account create/edit modals
- Ent schema regenerated for new usage_log fields

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/api_contract_test.go

@@ -401,6 +401,7 @@ func TestAPIContracts(t *testing.T) {
 							"first_token_ms": 50,
 							"image_count": 0,
 							"image_size": null,
+							"cache_ttl_overridden": false,
 							"created_at": "2025-01-02T03:04:05Z",
 							"user_agent": null
 						}