Prober: no claims under PROBE_MIN_FREE_GB free on the NVMe

At ~200 articles/s what the prober stores (inspections, par2 file lists,
the WAL) took ~0.8-1.1 GB an hour of homelab's root, and nothing stopped it
at the 10 GB floor. Below 12 GB free it claims nothing (resumed 1 GB above):
the lanes go dry and its share shrinks to its minimum.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

start/env.ts

@@ -78,6 +78,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   PREFETCH_PER_RANGE: Env.schema.number.optional(),
   PROBE_CONNECTIONS: Env.schema.number.optional(),
   PROBE_MIN_CONNECTIONS: Env.schema.number.optional(),
+  PROBE_MIN_FREE_GB: Env.schema.number.optional(),
   HTTP_ROUTES: Env.schema.enum.optional(['all', 'node', 'public'] as const),
   PREFETCH_PER_GROUP: Env.schema.number.optional(),
 })