feat(templates): source templates for multi-key balancing

A template stores every source field except name and api_key, so
operators spin up N key-bearing sources from one shared skeleton
instead of duplicating the whole source block N times.

- config: SourceTemplate type + RuntimeConfig.SourceTemplates stored
  in runtime.json alongside runtime sources
- store: UpsertTemplate / ListTemplates / RemoveTemplate
- core: Templates / SaveTemplate / RemoveTemplate
- gateway: GET/POST/DELETE /api/source_templates
- webui: source list gains a Templates button opening a manager with
  per-template edit/delete; the add-source dialog gains 'from
  template' (event-delegated picker) and 'as template' (card modal)
  buttons in its header; z-index fixed so the template editor layers
  above the manager

internal/config/config.go

@@ -314,12 +314,31 @@ func (c *Config) ApplyDefaults() error {
 
 // RuntimeConfig is the legacy runtime file format (kept for migration only).
 type RuntimeConfig struct {
-	Sources         []Source     `json:"sources,omitempty"`
-	DeletedSources  []string     `json:"deleted_sources,omitempty"`
-	DeletedAdapters []string     `json:"deleted_adapters,omitempty"`
-	Keys            []GWKey      `json:"keys,omitempty"`
-	Auto            []ModelScope `json:"auto,omitempty"`
-	AutoImage       []ModelScope `json:"auto_image,omitempty"`
+	Sources          []Source          `json:"sources,omitempty"`
+	SourceTemplates  []SourceTemplate  `json:"source_templates,omitempty"`
+	DeletedSources   []string          `json:"deleted_sources,omitempty"`
+	DeletedAdapters  []string          `json:"deleted_adapters,omitempty"`
+	Keys             []GWKey           `json:"keys,omitempty"`
+	Auto             []ModelScope      `json:"auto,omitempty"`
+	AutoImage        []ModelScope      `json:"auto_image,omitempty"`
+}
+
+// SourceTemplate stores reusable source configuration (everything except
+// name and api_key) so the WebUI can spin up multiple key-bearing sources
+// from one shared template.
+type SourceTemplate struct {
+	Name          string                 `json:"name"` 
+	BaseURL       string                 `json:"base_url"` 
+	Adapter       string                 `json:"adapter"` 
+	Endpoint      string                 `json:"endpoint,omitempty"` 
+	ImageEndpoint string                 `json:"image_endpoint,omitempty"` 
+	Models        []Model                `json:"models"` 
+	Headers       map[string]string      `json:"headers,omitempty"` 
+	Meta          map[string]interface{} `json:"meta,omitempty"` 
+	Temperature   float64                `json:"temperature,omitempty"` 
+	MaxTokens     int                    `json:"max_tokens,omitempty"` 
+	MaxConcurrent int                    `json:"max_concurrent,omitempty"` 
+	RPM           int                    `json:"rpm,omitempty"` 
 }
 
 // GWKey is a gateway API key persisted in the config file. Role is "admin"

internal/config/store.go

@@ -138,6 +138,49 @@ func (s *Store) DeletedAdapters() map[string]bool {
 	return out
 }
 
+// UpsertTemplate adds or replaces a source template and persists.
+func (s *Store) UpsertTemplate(t SourceTemplate) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i := range s.data.SourceTemplates {
+		if s.data.SourceTemplates[i].Name == t.Name {
+			s.data.SourceTemplates[i] = t
+			return s.persistLocked()
+		}
+	}
+	s.data.SourceTemplates = append(s.data.SourceTemplates, t)
+	return s.persistLocked()
+}
+
+// ListTemplates returns the saved source templates.
+func (s *Store) ListTemplates() []SourceTemplate {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	out := make([]SourceTemplate, len(s.data.SourceTemplates))
+	copy(out, s.data.SourceTemplates)
+	return out
+}
+
+// RemoveTemplate deletes a source template by name and persists.
+func (s *Store) RemoveTemplate(name string) (bool, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	kept := s.data.SourceTemplates[:0]
+	removed := false
+	for _, t := range s.data.SourceTemplates {
+		if t.Name == name {
+			removed = true
+			continue
+		}
+		kept = append(kept, t)
+	}
+	s.data.SourceTemplates = kept
+	if removed {
+		return true, s.persistLocked()
+	}
+	return false, nil
+}
+
 // LoadLegacy returns the full legacy RuntimeConfig from the file (used for
 // one-time migration into config.yaml). Returns nil if file is missing.
 func (s *Store) LoadLegacy() *RuntimeConfig {