Switch MCP transport from stdio to Streamable HTTP

Mount POST /mcp on the main Express app (stateless Streamable HTTP).
Keep optional MCP-only HTTP process via npm run mcp. Update docs.

.env.example

@@ -3,6 +3,10 @@ BASE_URL=http://webshare-api:3001
 WEBSHARE_USERNAME=
 WEBSHARE_PASSWORD=
 
+# MCP Streamable HTTP (mounted on the main app at POST /mcp)
+# MCP_ENABLED=true
+# MCP_PATH=/mcp
+
 # Host path mounted into the container (compose.yaml)
 # DOWNLOAD_HOST_PATH=./data/webshare