add: live encode progress tracking and read-only status web UI

Stream ffmpeg -progress during the video encode into an in-memory tracker
(internal/status) so the long-running step is no longer a black box: percent,
fps, speed, and ETA are derived from the source duration and updated ~1/s.
Progress prints to stdout only (no logs.db row) to avoid burying real events.

Expose it over HTTP (internal/server, default :8080, set http_addr to "" to
disable): GET /status returns the live snapshot, the pending input queue, and
recent non-debug events; GET / serves an embedded dashboard that polls /status
every second. The server shuts down on the same SIGINT/SIGTERM context as the
watcher.

internal/config/config.go

@@ -79,6 +79,10 @@ func Load(configPath string) (*types.Config, error) {
 	if cfg.LogRetentionDays == 0 {
 		cfg.LogRetentionDays = 7
 	}
+	if cfg.HTTPAddr == nil {
+		def := ":8080"
+		cfg.HTTPAddr = &def
+	}
 
 	return &cfg, nil
 }