Collect the logs of the local Docker containers

- docker.go follows every running container through the Docker API
  (events + logs with follow), resumes after a restart from the last
  position saved in /data/docker-state.json, reads DOCKER_BACKFILL (1h)
  of history for new containers, strips terminal color codes and guesses
  the severity from the line (JSON, logfmt, [ERROR], ERROR ...).
- Logs carry source_type=docker, container, container_id, image,
  compose_project, compose_service and stream; host is the Docker host.
- Settings > Sources: one switch per container (grouped by compose
  project), enable/disable all, follow new containers automatically.
  Choices are saved per compose service in /data/docker.json.
- Source filter (syslog / docker) in the filter bar and the live view.
- docker-compose: read-only docker-socket-proxy; Logstream and the proxy
  are labelled logstream.exclude=true and never collected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

export.go

@@ -22,6 +22,9 @@ var exportColumns = []struct{ field, name string }{
 	{"procid", "pid"},
 	{"source", "source_ip"},
 	{"proto", "proto"},
+	{"source_type", "source_type"},
+	{"container", "container"},
+	{"image", "image"},
 	{"_msg", "message"},
 }