feat: make the refresh interval configurable

[TyceHerrman]

Summary

Allow users to reduce Satpaper's bandwidth usage by configuring how long it waits between checks for new imagery.

The new --sleep-duration/SATPAPER_SLEEP_DURATION setting is expressed in seconds, defaults to 60, and rejects zero.

Changes

• add a positive u64 sleep-duration CLI and environment setting
• convert the configured seconds to a Duration once when the update loop starts
• document the setting in the options reference
• add the setting to the systemd, launchd, and Docker Compose examples
• cover the default, CLI override, and zero-value rejection

Testing

• cargo test --release -- --skip generate_wallpaper — 3 passed
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

Closes #28