feat(scheduler): separate image-generation AUTO chain with UI toggle

Image models previously could not be scheduled through a priority
chain: the chat AUTO chain explicitly skips image-kind slots, and
AUTO image requests fell back to unordered registry discovery.

- config: add auto_image rules (auto_image yaml / image_rules json);
  legacy auto rules keep their meaning as the chat chain
- core: buildAutoImageChain mirrors buildAutoChain with inverted kind
  filter (image-only); SaveAutoImageRules + AutoImageRules/AutoImageChain
- scheduler: ChainImage walks the chain tier-by-tier with round-robin
  and preference ordering, skipping cooling slots
- gateway: handleImage AUTO now runs down AutoImageChain when one is
  configured (falls back to legacy discovery otherwise) and records
  the actual served model; handleAutoAPI GET returns image_rules and
  PUT accepts image_rules independently of rules
- webui: priority page gains a chat/image toggle editing two
  independent lane sets; add-slot picker filters by active kind;
  persistAuto writes only the active chain's field

internal/config/config.go

@@ -24,7 +24,8 @@ type Config struct {
 	TLSKeyFile    string       `yaml:"tls_key_file,omitempty"`    // PEM private key
 	PublicBaseURL string       `yaml:"public_base_url,omitempty"` // external base for generated config snippets; default inferred from request
 	Sources       []Source     `yaml:"sources"`
-	Auto          []ModelScope `yaml:"auto,omitempty"`            // AUTO 调度链规则（WebUI 优先级页编辑）
+	Auto          []ModelScope `yaml:"auto,omitempty"`            // AUTO 调度链规则（WebUI 优先级页编辑，chat）
+	AutoImage     []ModelScope `yaml:"auto_image,omitempty"`      // AUTO 生图调度链规则（WebUI 优先级页·生图）
 	Keys          []GWKey      `yaml:"keys,omitempty"`            // 网关密钥（WebUI 密钥页管理）
 }
 
@@ -318,6 +319,7 @@ type RuntimeConfig struct {
 	DeletedAdapters []string     `json:"deleted_adapters,omitempty"`
 	Keys            []GWKey      `json:"keys,omitempty"`
 	Auto            []ModelScope `json:"auto,omitempty"`
+	AutoImage       []ModelScope `json:"auto_image,omitempty"`
 }
 
 // GWKey is a gateway API key persisted in the config file. Role is "admin"