Update Scheduling (per-feed auto-update, no wasted resources)

Each feed can have its own update schedule; the program only fetches when the feed is "due". Expressions: interval type 5m / 30m / 1h / 7d, fixed time daily@10:00, weekly@Mon 08:00, manual manual.

sip --schedule 1 30m            # update feed 1 every 30 minutes
sip --schedule 2 daily@10:00    # update feed 2 at 10:00 daily
sip --schedule 3 manual         # set feed 3 to manual
sip -l                          # each feed shows "frequency · last · next"

• When the program opens: silently syncs all due feeds
• While the program is running: checks every 15 minutes in the background, updating only what's due
• In CLI mode there's no auto-sync, but it reminds you when feeds are due (--ignoresafeannouncement doesn't suppress this reminder; --json mode auto-suppresses it to avoid polluting structured output)
• Due determination: now >= last fetch time + schedule due point; each successful fetch rewrites the "last fetch time"