Read feed lines up to 1 MiB (closes #76)

The streamer read the feed with a bufio.Scanner at its default limit of
64 KiB per line, so a longer line ended the connection and the updates sent
while it reconnected were lost. The scanner now accepts lines up to 1 MiB
(maxLineSize), well above an extended BGP message written out as the feed
writes it.

Model: opus-5-5

internal/streamer/streamer.go

@@ -53,7 +53,8 @@ const (
 	metricsLogInterval    = 10 * time.Second
 	bytesPerKB            = 1024
 	bytesPerMB            = 1024 * 1024
-	maxConcurrentHandlers = 800 // Maximum number of concurrent message handlers
+	maxLineSize           = bytesPerMB // Well above an extended BGP message (RFC 8654) in hex plus its prefixes
+	maxConcurrentHandlers = 800        // Maximum number of concurrent message handlers
 
 	// Backpressure constants
 	backpressureThreshold = 0.5 // Start dropping at 50% queue utilization
@@ -573,6 +574,7 @@ func (s *Streamer) stream(ctx context.Context) error {
 	}()
 
 	scanner := bufio.NewScanner(reader)
+	scanner.Buffer(nil, maxLineSize)
 
 	for scanner.Scan() {
 		idleTimer.Reset(s.idleTimeout)