Add OCPP control for the Anker Solix EV charger (Own/Proxy CSMS)

The Anker Solix connector was read-only (cloud monitoring only). Add an
OCPP 1.6J control path with a per-user, cascading control mode:

  - off   monitoring only (default, unchanged behavior)
  - own   DriverVault is the charger's Central System (full control)
  - proxy DriverVault relays to Anker's cloud and injects commands

New internal/ocpp subsystem (stdlib-only, hand-rolled RFC 6455): a CSMS
with session management, inbound dispatch, and typed control commands
(RemoteStart/Stop, SetChargingProfile current limit, ChangeAvailability,
Reset, UnlockConnector, TriggerMessage, Get/ChangeConfiguration). Own- and
proxy-mode paths are verified end-to-end against a simulated charge point.

The charger connects to /ocpp/{serial}, authenticated with OCPP Basic auth
(serial + a per-charger control token) resolved to the owning user via an
in-memory token index. Control REST endpoints mirror the monitoring ones and
reuse the same cascade gate plus a live-session check. controlMode is a new
cascade field (global -> org -> user) advertised as a select on the plugin.

Frontend: control-mode select + provisioning card in Settings, and a real
Start/Stop/limit/reset control panel in Charging, gated on the active mode.

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

API Server/internal/ocpp/ocpp16_test.go

@@ -0,0 +1,64 @@
+package ocpp
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestDecodeCall(t *testing.T) {
+	m, err := DecodeMessage([]byte(`[2,"abc","BootNotification",{"chargePointModel":"A5191"}]`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if m.Type != MessageTypeCall || m.ID != "abc" || m.Action != "BootNotification" {
+		t.Fatalf("bad decode: %+v", m)
+	}
+	var p struct {
+		Model string `json:"chargePointModel"`
+	}
+	if err := json.Unmarshal(m.Payload, &p); err != nil || p.Model != "A5191" {
+		t.Fatalf("payload = %s (%v)", m.Payload, err)
+	}
+}
+
+func TestDecodeCallResultAndError(t *testing.T) {
+	r, err := DecodeMessage([]byte(`[3,"abc",{"status":"Accepted"}]`))
+	if err != nil || r.Type != MessageTypeCallResult {
+		t.Fatalf("callresult decode: %+v %v", r, err)
+	}
+	e, err := DecodeMessage([]byte(`[4,"abc","NotImplemented","nope",{}]`))
+	if err != nil || e.Type != MessageTypeCallError || e.ErrorCode != "NotImplemented" || e.ErrorDescription != "nope" {
+		t.Fatalf("callerror decode: %+v %v", e, err)
+	}
+}
+
+func TestEncodeRoundTrip(t *testing.T) {
+	b, err := EncodeCall("id1", "Reset", map[string]any{"type": "Soft"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m, err := DecodeMessage(b)
+	if err != nil || m.Action != "Reset" {
+		t.Fatalf("round trip: %+v %v", m, err)
+	}
+}
+
+func TestEncodeEmptyPayloadIsObject(t *testing.T) {
+	// OCPP requires the payload be a JSON object, never null.
+	b, err := EncodeCallResult("id1", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(b) != `[3,"id1",{}]` {
+		t.Fatalf("empty payload encoded as %s, want [3,\"id1\",{}]", b)
+	}
+}
+
+func TestDecodeRejectsGarbage(t *testing.T) {
+	if _, err := DecodeMessage([]byte(`{"not":"an array"}`)); err == nil {
+		t.Fatal("expected error decoding non-array")
+	}
+	if _, err := DecodeMessage([]byte(`[2,"id"]`)); err == nil {
+		t.Fatal("expected error decoding short CALL")
+	}
+}