Title: Document ffmpeg reconnect options as opt-in for unstable HTTP sources

Problem

Many IPTV providers periodically tear down long-lived HTTP stream connections (session rotation, load-balancer timeouts, CDN failover). With the current default ffmpeg options there is no in-process reconnect: when the provider closes the connection, ffmpeg exits, the whole buffer pipeline restarts, and client-side segmenters (Plex/Jellyfin live TV) render the discontinuity as a freeze or a short cut every time it happens.

ffmpeg's HTTP reconnect options address that class of drop:

-reconnect 1 -reconnect_streamed 1 -reconnect_on_network_error 1 -reconnect_on_http_error 4xx,5xx -reconnect_delay_max 2

With these, ffmpeg re-establishes the connection inside the same process, so the output timeline stays continuous instead of the buffer layer restarting the stream.

Field caveat (why this is opt-in, not a default)

Testing against a real provider surfaced a worse failure mode on some servers: on every new connection the server replays the last ~30 seconds of content as a catch-up backlog. With in-process reconnect, that replay gets spliced into the same continuous output timeline — the client visibly loops or jumps back over content it already played, on every reconnect.

Without the reconnect flags, a dropped connection makes ffmpeg exit and the buffer layer restarts the stream: a brief cut, but a clean timeline. On servers with backlog-replay behavior, that's the smaller failure of the two.

Since the right choice depends on how a given upstream server behaves after a dropped connection, and there's no reliable way to detect that ahead of time, this should not be an unconditional default for every install.

Change (revised from the original proposal)

The original version of this PR added the reconnect flags directly to System.FFmpeg.DefaultOptions. This revision does not change the default — System.FFmpeg.DefaultOptions is left exactly as it ships today. Instead, src/config.go gets a comment next to the default documenting the reconnect flags, when to add them, and the backlog-replay caveat above, so operators can opt in deliberately after confirming their source doesn't replay backlog on reconnect (Settings > FFmpeg options).

Effect on existing/new installs

None — this is a documentation-only change (a code comment). No behavior changes for any install, new or existing.

Related: #659 touches the same default string; unaffected by this change since the default itself is untouched here.