chat: rename the pi chat transport + model list to chat (Stage 4b/1)

Pure rename, no behavior change. Moves the misnamed "pi" chat harness into the
chat namespace:

- api/pi/{websocket,session-manager,types,logger,list-models} → api/chat/
- merge api/pi/rest.ts into api/chat/chat.ts (/pi/models → /chat/models,
  /pi/stt → /chat/stt); drop the piRestRouter mount
- PiEvent → ChatEvent, piWebsocket → chatWebsocket, listPiModels → listChatModels
- WS route /api/pi/chat/ws → /api/chat/ws, provider tag 'pi' → 'chat'
- frontend: useChat/useAudioRecording URLs, usePiModels→useModels /
  useVisiblePiModels→useVisibleModels / useEnabledPiModels→useEnabledModels,
  'PI_MODELS' query key → 'CHAT_MODELS', attachments provider 'pi-mono' → 'chat'

The /pi-mono provider/harness settings router is renamed separately (next commit).
Note: the WS route change requires the mobile app to point at /api/chat/ws.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/api/chat/logger.ts

@@ -0,0 +1,62 @@
+/**
+ * Structured logging utility for Pi harness
+ */
+
+export type LogLevel = 'DEBUG' | 'INFO' | 'WARN' | 'ERROR';
+
+type LogContext = {
+  sessionId?: string;
+  email?: string;
+  model?: string;
+  requestId?: string;
+  [key: string]: unknown;
+};
+
+const LOG_COLORS = {
+  DEBUG: '\x1b[36m', // Cyan
+  INFO: '\x1b[32m', // Green
+  WARN: '\x1b[33m', // Yellow
+  ERROR: '\x1b[31m', // Red
+  RESET: '\x1b[0m',
+};
+
+function formatTimestamp(): string {
+  return new Date().toISOString();
+}
+
+function formatContext(context?: LogContext): string {
+  if (!context || Object.keys(context).length === 0) return '';
+  
+  const lines = Object.entries(context)
+    .map(([key, value]) => `  ${key}=${value}`)
+    .join('\n');
+  
+  return '\n' + lines;
+}
+
+function log(level: LogLevel, message: string, context?: LogContext) {
+  const timestamp = formatTimestamp();
+  const color = LOG_COLORS[level];
+  const reset = LOG_COLORS.RESET;
+  const contextStr = formatContext(context);
+
+  console.log(`${color}[${timestamp}] [Pi] [${level}]${reset} ${message}${contextStr}`);
+}
+
+export const logger = {
+  debug(message: string, context?: LogContext) {
+    log('DEBUG', message, context);
+  },
+
+  info(message: string, context?: LogContext) {
+    log('INFO', message, context);
+  },
+
+  warn(message: string, context?: LogContext) {
+    log('WARN', message, context);
+  },
+
+  error(message: string, context?: LogContext) {
+    log('ERROR', message, context);
+  },
+};