Add naming scheme selection to run configuration

Add a nullable naming_scheme column to NuzlockeRun so users can pick a
themed word category for nickname suggestions. Includes Alembic migration,
updated Pydantic schemas, a GET /runs/naming-categories endpoint backed by
a cached dictionary loader, and frontend dropdowns in both the NewRun
creation flow and the RunDashboard for mid-run changes.

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

frontend/src/api/runs.ts

@@ -28,3 +28,7 @@ export function updateRun(
 export function deleteRun(id: number): Promise<void> {
   return api.del(`/runs/${id}`)
 }
+
+export function getNamingCategories(): Promise<string[]> {
+  return api.get('/runs/naming-categories')
+}