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/dto/provider.go

@@ -13,6 +13,7 @@ type ProviderDTO struct {
 	Group          string    `json:"group"`
 	Models         []string  `json:"models"` // List of supported model names
 	Status         string    `json:"status"`
+	Weight         int       `json:"weight,omitempty"`
 	AutoBan        *bool     `json:"auto_ban,omitempty"`
 	BanReason      string    `json:"ban_reason,omitempty"`
 	BanUntil       time.Time `json:"ban_until,omitempty"`

internal/dto/provider_create.go

@@ -0,0 +1,35 @@
+package dto
+
+// ProviderPresetCreateDTO creates an official provider with sensible defaults.
+// For preset providers, base_url is derived automatically; users typically only provide api_key.
+type ProviderPresetCreateDTO struct {
+	Preset string `json:"preset"` // openai | anthropic | gemini
+
+	// Optional fields.
+	Name   string   `json:"name"`
+	Group  string   `json:"group"`
+	Models []string `json:"models"`
+
+	APIKey         string `json:"api_key"`
+	GoogleProject  string `json:"google_project,omitempty"`
+	GoogleLocation string `json:"google_location,omitempty"`
+
+	Status  string `json:"status"`
+	Weight  int    `json:"weight,omitempty"`
+	AutoBan *bool  `json:"auto_ban,omitempty"`
+}
+
+// ProviderCustomCreateDTO creates an OpenAI-compatible provider.
+// For custom providers, base_url is required.
+type ProviderCustomCreateDTO struct {
+	Name   string   `json:"name"`
+	Group  string   `json:"group"`
+	Models []string `json:"models"`
+
+	BaseURL string `json:"base_url"`
+	APIKey  string `json:"api_key"`
+
+	Status  string `json:"status"`
+	Weight  int    `json:"weight,omitempty"`
+	AutoBan *bool  `json:"auto_ban,omitempty"`
+}