fix(nntp): handle open-ended OVER range like '5-'

[forgou37]

Problem

The NNTP OVER command with an open-ended range like 5- (meaning "all articles from 5 to end") currently returns 0 articles.

Root cause: parseRange("5-") splits on - → ["5", ""]. strconv.ParseInt("", 10, 64) returns an error, so the function falls through to return 0, 0 instead of returning (5, math.MaxInt64).

Fix

Check for empty parts[1] before attempting to parse it — treat it as an open-ended range (math.MaxInt64).

OVER 5-   →  articles 5..MaxInt64  ✓  (was returning empty)
OVER 5-10 →  articles 5..10       ✓  (unchanged)
OVER 5    →  articles 5..5        ✓  (unchanged)
OVER      →  articles 0..MaxInt64 ✓  (unchanged)

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