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/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 {