feat: add parse endpoint, refactor recurring tasks, and improve web task completion

Extract CreateRecurringTask into engine package for reuse by both CLI
and API. Add POST /tasks/parse endpoint for CLI-style input parsing.
Remove FK constraint on change_log to preserve history after task
deletion. Update web frontend to filter completed tasks from view and
add mock mode support for development.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/api/server.go

@@ -53,6 +53,7 @@ func (s *Server) setupRoutes() {
 		r.Route("/tasks", func(r chi.Router) {
 			r.Get("/", handlers.ListTasks)
 			r.Post("/", handlers.CreateTask)
+			r.Post("/parse", handlers.ParseTask)
 			r.Get("/{uuid}", handlers.GetTask)
 			r.Put("/{uuid}", handlers.UpdateTask)
 			r.Delete("/{uuid}", handlers.DeleteTask)