Rebuild: spread prefetch connections over ranges and groups

At most PREFETCH_PER_RANGE (2) connections on a range and
PREFETCH_PER_GROUP (4) on a group; SPOOL_AHEAD_PER_LEASE 2 -> 4 so there
are targets enough. The report says how many ranges and groups are in
flight.

Homelab, 40 connections, alternated 12 min each: unlimited (~10 ranges
in flight) 281k and 167k articles/s; spread (~22 ranges) 319k and 296k,
chunks 11.5 s against 13-24 s.

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

start/env.ts

@@ -62,4 +62,6 @@ export default await Env.create(new URL('../', import.meta.url), {
   SPOOL_AHEAD_PER_LEASE: Env.schema.number.optional(),
   SPOOL_MIN_AHEAD: Env.schema.number.optional(),
   SPOOL_MAX_GB: Env.schema.number.optional(),
+  PREFETCH_PER_RANGE: Env.schema.number.optional(),
+  PREFETCH_PER_GROUP: Env.schema.number.optional(),
 })