Rebuild: give up stalled requests, not slow ones

a.b.0day.stuffz sends steadily but slowly (~400 kB a minute, 5-7
minutes per 100k chunk): the fixed deadline (90 s, doubling up to 360 s)
gave its chunks up again and again, and two of its ranges' worker
streams timed out. A stall is a request that got less than
NNTP_STALL_KB_PER_MINUTE (100) in the last minute (the stuck replies
got ~25 kB a minute); then its connection is closed and the chunk
fetched again. Replaces NNTP_CHUNK_DEADLINE_SECONDS.

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

start/env.ts

@@ -47,7 +47,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   NNTP_HEDGE_AFTER_SECONDS: Env.schema.number.optional(),
   NNTP_HEDGE_CONNECTIONS: Env.schema.number.optional(),
   NNTP_CONNECTION_MAX_MINUTES: Env.schema.number.optional(),
-  NNTP_CHUNK_DEADLINE_SECONDS: Env.schema.number.optional(),
+  NNTP_STALL_KB_PER_MINUTE: Env.schema.number.optional(),
 
   // Newznab API key (empty: no key needed)
   NEWZNAB_API_KEY: Env.schema.secret.optional(),