Add log tail command for live log streaming

This commit adds a new `ctrld log tail` subcommand that streams
runtime debug logs to the terminal in real-time, similar to `tail -f`.

Changes:
- log_writer.go: Add Subscribe/tailLastLines for fan-out to tail clients
- control_server.go: Add /log/tail endpoint with streaming response
  - Internal logging: subscribes to logWriter for live data
  - File-based logging: polls log file for new data (200ms interval)
  - Sends last N lines as initial context on connect
- commands.go: Add `log tail` cobra subcommand with --lines/-n flag
- control_client.go: Add postStream() with no timeout for long-lived connections

Usage:
  sudo ctrld log tail          # shows last 10 lines then follows
  sudo ctrld log tail -n 50    # shows last 50 lines then follows
  Ctrl+C to stop

cmd/cli/control_client.go

@@ -32,6 +32,12 @@ func (c *controlClient) post(path string, data io.Reader) (*http.Response, error
 	return c.c.Post("http://unix"+path, contentTypeJson, data)
 }
 
+// postStream sends a POST request with no timeout, suitable for long-lived streaming connections.
+func (c *controlClient) postStream(path string, data io.Reader) (*http.Response, error) {
+	c.c.Timeout = 0
+	return c.c.Post("http://unix"+path, contentTypeJson, data)
+}
+
 // deactivationRequest represents request for validating deactivation pin.
 type deactivationRequest struct {
 	Pin int64 `json:"pin"`