Add NexusAI: universal MCP service (TTS/STT/image/video/music/web-search)

Monorepo-lite with shared @nexusai/core and @nexusai/mcp-server (stdio).
Provider-agnostic tools with explicit provider selection; NordRouter adapter
(media generate/poll/download, estimate, upload, models) and search via
perplexity/sonar. Local STT via faster-whisper uv sidecar. SQLite journal of
every generation for usage stats and future admin. 10 MCP tools.

NexusAI/packages/core/src/shared/logger.ts

@@ -0,0 +1,42 @@
+/**
+ * MCP servers communicate over stdio, so ALL logs must go to stderr.
+ * Never write logs to stdout — it corrupts the JSON-RPC stream.
+ */
+type Level = 'debug' | 'info' | 'warn' | 'error';
+
+const LEVELS: Record<Level, number> = { debug: 10, info: 20, warn: 30, error: 40 };
+
+function currentThreshold(): number {
+  const env = (process.env.NEXUS_LOG_LEVEL || 'info').toLowerCase() as Level;
+  return LEVELS[env] ?? LEVELS.info;
+}
+
+function emit(level: Level, scope: string, message: string, extra?: unknown): void {
+  if (LEVELS[level] < currentThreshold()) return;
+  const ts = new Date().toISOString();
+  let line = `[${ts}] ${level.toUpperCase()} (${scope}) ${message}`;
+  if (extra !== undefined) {
+    try {
+      line += ` ${typeof extra === 'string' ? extra : JSON.stringify(extra)}`;
+    } catch {
+      line += ' [unserializable extra]';
+    }
+  }
+  process.stderr.write(line + '\n');
+}
+
+export interface Logger {
+  debug(message: string, extra?: unknown): void;
+  info(message: string, extra?: unknown): void;
+  warn(message: string, extra?: unknown): void;
+  error(message: string, extra?: unknown): void;
+}
+
+export function createLogger(scope: string): Logger {
+  return {
+    debug: (m, e) => emit('debug', scope, m, e),
+    info: (m, e) => emit('info', scope, m, e),
+    warn: (m, e) => emit('warn', scope, m, e),
+    error: (m, e) => emit('error', scope, m, e),
+  };
+}