SSE EventSource connections for AIS, ACARS, VDL2, and radiosonde were
not closed when switching modes, causing fd exhaustion after repeated
switches. Also fixes socket leaks on exception paths in AIS/ADS-B
stream parsers, closes subprocess pipes in safe_terminate/cleanup, and
caches skyfield timescale at module level to avoid per-request fd churn.
Closes#169
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>