fix: add missing TemplateContext type definition

The type was used throughout services/context but never defined —
likely lost during upstream merge. It's a map[string]any that
implements context.Context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/context/context_template.go

@@ -17,6 +17,10 @@ import (
 	"code.gitea.io/gitea/services/webtheme"
 )
 
+// TemplateContext is a map that holds template rendering context data
+// and implements context.Context for passing request-scoped values.
+type TemplateContext map[string]any
+
 var _ context.Context = TemplateContext(nil)
 
 func NewTemplateContext(ctx context.Context, req *http.Request) TemplateContext {