Add endpoint deletion

The endpoints screen could only create and edit servers, so a mistyped
or retired endpoint stayed in the list forever.

Tasks reference endpoints without ON DELETE CASCADE, so a referenced
endpoint is refused with 409 and a count of the tasks using it rather
than cascading away migration history. The foreign-key violation is
mapped to the same status to cover a task created between check and
delete.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/httpapi/endpoints.go

@@ -2,8 +2,11 @@ package httpapi
 
 import (
 	"encoding/json"
+	"errors"
+	"fmt"
 	"net/http"
 
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/vasyansk/imap-copier/internal/store"
 )
 
@@ -54,6 +57,36 @@ func (s *Server) handleUpdateEndpoint(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusNoContent)
 }
 
+// handleDeleteEndpoint removes an endpoint that no task references. A referenced
+// endpoint is refused with 409 rather than a foreign-key error, and the same
+// status covers the race where a task is created between check and delete.
+func (s *Server) handleDeleteEndpoint(w http.ResponseWriter, r *http.Request) {
+	id, err := pathID(r, "id")
+	if err != nil {
+		http.Error(w, "bad id", http.StatusBadRequest)
+		return
+	}
+	used, err := s.store.CountTasksUsingEndpoint(r.Context(), id)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if used > 0 {
+		http.Error(w, fmt.Sprintf("endpoint is used by %d task(s) — delete them first", used), http.StatusConflict)
+		return
+	}
+	if err := s.store.DeleteEndpoint(r.Context(), id); err != nil {
+		var pgErr *pgconn.PgError
+		if errors.As(err, &pgErr) && pgErr.Code == "23503" {
+			http.Error(w, "endpoint is used by a task — delete it first", http.StatusConflict)
+			return
+		}
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
 func (s *Server) handleListEndpoints(w http.ResponseWriter, r *http.Request) {
 	eps, err := s.store.ListEndpoints(r.Context())
 	if err != nil {