fix: WebSSH terminal echo delay — replace readline() with read()

asyncssh's async-for on shell.stdout internally calls readline()
which buffers until a newline is found. For interactive terminal
use, single-character echo never contains a newline, so all
typed characters were buffered until Enter was pressed.

Fix: use shell.stdout.read(4096) which returns data as soon as
any is available, and set bufsize=4096 on create_process() to
reduce the internal read buffer from 128KB to 4KB for low-latency
interactive echo.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/api/webssh.py

@@ -180,6 +180,7 @@ async def terminal_ws(
             shell_proc = conn.create_process(
                 term_type="xterm-256color",
                 term_size=(24, 80),
+                bufsize=4096,  # Small buffer for low-latency interactive echo
             )
         except Exception as e:
             # Stale pooled connection — force-close and retry with fresh connection
@@ -190,6 +191,7 @@ async def terminal_ws(
                 shell_proc = conn.create_process(
                     term_type="xterm-256color",
                     term_size=(24, 80),
+                    bufsize=4096,  # Small buffer for low-latency interactive echo
                 )
             except Exception as e2:
                 logger.error(f"WebSSH shell creation failed on fresh connection (server={server.id}): {type(e2).__name__}: {e2!r}")
@@ -218,13 +220,21 @@ async def terminal_ws(
             command_buffer = ""  # Buffer for command logging
 
             async def _read_shell_output():
-                """Read SSH shell output -> send to WebSocket client"""
+                """Read SSH shell output -> send to WebSocket client
+
+                Uses read() instead of async-for to avoid readline() buffering.
+                async-for calls readline() internally, which waits for newline
+                characters before yielding — causing echo delay in interactive
+                terminal use.  read(n) returns data as soon as any is available.
+                """
                 try:
-                    async for data in shell.stdout:
-                        await websocket.send_json({
-                            "type": MSG_DATA,
-                            "data": data,
-                        })
+                    while not shell.stdout.at_eof():
+                        data = await shell.stdout.read(4096)
+                        if data:
+                            await websocket.send_json({
+                                "type": MSG_DATA,
+                                "data": data,
+                            })
                 except Exception:
                     pass