feat(api): add provider creation endpoints and weight support

- Add `POST /admin/providers/preset` for streamlined creation of official providers (OpenAI, Anthropic, Gemini)
- Add `POST /admin/providers/custom` for generic OpenAI-compatible providers
- Add `weight` field to provider model and DTOs to enable weighted routing
- Update sync service to propagate provider weights
- Add unit tests for new creation handlers

internal/api/provider_create_handler_test.go

@@ -0,0 +1,70 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/ez-api/ez-api/internal/model"
+	"github.com/gin-gonic/gin"
+)
+
+func TestCreateProviderPreset_OpenAI_SetsBaseURL(t *testing.T) {
+	h, _ := newTestHandler(t)
+
+	r := gin.New()
+	r.POST("/admin/providers/preset", h.CreateProviderPreset)
+
+	reqBody := map[string]any{
+		"preset":  "openai",
+		"api_key": "k",
+		"models":  []string{"gpt-4o-mini"},
+	}
+	b, _ := json.Marshal(reqBody)
+
+	req := httptest.NewRequest(http.MethodPost, "/admin/providers/preset", bytes.NewReader(b))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+	r.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusCreated {
+		t.Fatalf("expected 201, got %d body=%s", rr.Code, rr.Body.String())
+	}
+	var got model.Provider
+	if err := json.Unmarshal(rr.Body.Bytes(), &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if got.Type != "openai" {
+		t.Fatalf("expected type openai, got %q", got.Type)
+	}
+	if got.BaseURL != "https://api.openai.com" {
+		t.Fatalf("expected base_url=https://api.openai.com, got %q", got.BaseURL)
+	}
+	if got.Name == "" {
+		t.Fatalf("expected generated name")
+	}
+}
+
+func TestCreateProviderCustom_RequiresBaseURL(t *testing.T) {
+	h, _ := newTestHandler(t)
+
+	r := gin.New()
+	r.POST("/admin/providers/custom", h.CreateProviderCustom)
+
+	reqBody := map[string]any{
+		"name":    "c1",
+		"api_key": "k",
+	}
+	b, _ := json.Marshal(reqBody)
+
+	req := httptest.NewRequest(http.MethodPost, "/admin/providers/custom", bytes.NewReader(b))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+	r.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusBadRequest {
+		t.Fatalf("expected 400, got %d body=%s", rr.Code, rr.Body.String())
+	}
+}