api: optimize status endpoint payload and caching

docs/API.md

@@ -149,7 +149,12 @@ Returns API and feature discovery data for clients.
 
 Returns complete controller status.
 
-Top-level response:
+Example requests:
+
+    GET /api/v1/status
+    GET /api/v1/status?fields=battery,relays,temps
+
+Top-level full response:
 
 ```json
 {
@@ -166,6 +171,40 @@ Top-level response:
 }
 ```
 
+#### Optional `fields` query parameter
+
+`GET /api/v1/status` accepts an optional comma-separated `fields` query parameter.
+
+Valid fields:
+
+    battery, temps, relays, vehicle, network, alarms, system, config
+
+If `fields` is omitted, the endpoint returns the complete status payload.
+
+If an unknown field is requested, the Cargo ESP32 returns HTTP `400`:
+
+    {
+      "ok": false,
+      "error": "invalid_field",
+      "details": ["Unknown field 'xyz'"]
+    }
+
+#### HTTP caching
+
+`GET /api/v1/status` returns these cache validation headers:
+
+    ETag
+    Last-Modified
+    Cache-Control: no-cache
+
+Clients may revalidate with `If-None-Match` or `If-Modified-Since`.
+
+If the selected status payload has not changed, the Cargo ESP32 returns:
+
+    HTTP/1.1 304 Not Modified
+
+A `304` response has no JSON body.
+
 ### battery
 
 ```json