feat(panel): let a sponsor slot start at a scheduled time

sponsors.json only had an end date, so a booked placement had to be
added to the file on the day it started. An optional `from` now hides
a sponsor (and its logo) until that instant; entries without it show
immediately as before.

frontend/src/generated/schemas.ts

@@ -4476,6 +4476,12 @@ export const SCHEMAS: Record<string, unknown> = {
         "nullable": true,
         "type": "boolean"
       },
+      "from": {
+        "example": "2026-10-01T00:00:00Z",
+        "format": "date-time",
+        "nullable": true,
+        "type": "string"
+      },
       "id": {
         "example": "acme-2026-10",
         "type": "string"