sxm.pl: persist channel list to disk cache with expiry and background refresh

[Copilot]

The proxy fetched the channel list from the SiriusXM API on every cold start and never persisted it, adding unnecessary latency and API calls. This adds a full cache lifecycle: save on fetch, load at startup, and async background refresh on expiry.

Cache format

channels.json is written to the same directory as the cookie file:

{
  "fetched_at": 1740000000,
  "expires_at": 1740086400,
  "channels": [ ... ]
}

Changes

• CHANNEL_CACHE_TTL — new constant (86400 s / 24 h)
• SiriusXM::new() — derives channel_cache_file path (<cookie_dir>/channels.json), initialises channel_cache_expires, calls load_channel_cache() at construction time
• load_channel_cache() — reads and validates channels.json on startup; if the cache is expired it still populates $self->{channels} so requests are served immediately and sets channel_cache_expires = 0 to trigger an immediate background refresh
• save_channel_cache() — serialises the channel array with fetched_at/expires_at envelope; updates in-memory expiry
• get_channels() — calls save_channel_cache() after every successful API fetch
• refresh_channel_cache_if_expired() — background refresh called from the server loop; keeps old channel list in place while fetching, falls back to old data on failure and schedules a 5-minute retry
• start_http_daemon() loop — wires refresh_channel_cache_if_expired() alongside the existing refresh_expired_playlists() / process_segment_queues() calls

Original prompt

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This section details on the original issue you should resolve

<issue_title>Save channel list to cache</issue_title>
<issue_description>In the proxy sxm.pl

Save channel listing to the cache. Make sure its in a format where we know the expiry
Use the same folder we are using for cookies

Load the channel listing from cache on startup
Run a background event to refresh the cache when it expires. Do this asynchronously.
Continue to use and serve channel data until the new new channel data arrives.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Save channel list to cache #69

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