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/core/core.go

@@ -675,6 +675,33 @@ func (c *Core) Sources() []config.Source {
 	return c.mergedSources()
 }
 
+// Templates returns the saved source templates.
+func (c *Core) Templates() []config.SourceTemplate {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.store.ListTemplates()
+}
+
+// SaveTemplate upserts a source template.
+func (c *Core) SaveTemplate(t config.SourceTemplate) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if t.Name == "" {
+		return fmt.Errorf("template requires a name")
+	}
+	return c.store.UpsertTemplate(t)
+}
+
+// RemoveTemplate deletes a source template by name.
+func (c *Core) RemoveTemplate(name string) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if _, err := c.store.RemoveTemplate(name); err != nil {
+		return err
+	}
+	return nil
+}
+
 func normalizeSource(s *config.Source) error {
 	if s.Name == "" || s.BaseURL == "" {
 		return fmt.Errorf("source requires name and base_url")