Rebuild: hedged chunk fetches and connection max age (both off by default)

A chunk not in after NNTP_HEDGE_AFTER_SECONDS (10) is asked again on one
of NNTP_HEDGE_CONNECTIONS spare connections; the first answer wins and
the other connection is closed (lib/nntp/hedge.ts, tested). Connections
can be retired after NNTP_CONNECTION_MAX_MINUTES (±25%). The prefetch
report counts hedges and slow first requests by connection age.

Homelab, 15 min each: hedging on (40 + 6) 280k articles/s, off 273k.
The second request won about half of its races, but ~25-35% of first
requests were slow (>10 s) at every connection age, so neither helps:
Eweka is slow under our load, not on particular connections. Both
off by default; the age statistics stay.

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

start/env.ts

@@ -44,6 +44,9 @@ export default await Env.create(new URL('../', import.meta.url), {
   NNTP_SECURE: Env.schema.boolean.optional(),
   NNTP_CONNECTIONS: Env.schema.number.optional(),
   NNTP_TIMEOUT_MS: Env.schema.number.optional(),
+  NNTP_HEDGE_AFTER_SECONDS: Env.schema.number.optional(),
+  NNTP_HEDGE_CONNECTIONS: Env.schema.number.optional(),
+  NNTP_CONNECTION_MAX_MINUTES: Env.schema.number.optional(),
 
   // Newznab API key (empty: no key needed)
   NEWZNAB_API_KEY: Env.schema.secret.optional(),