fix(api): add snake_case json tags to Endpoint/Task/request bodies for frontend contract

Go's encoding/json does not bridge snake_case <-> PascalCase field names,
so store.Endpoint, store.Task and the anonymous request bodies in
accounts.go/auth.go were silently decoding empty/zero values from the
frontend's snake_case JSON contract (tls_mode, role_label,
src_endpoint_id, dst_endpoint_id, src_login/pass, dst_login/pass).
Adds explicit json tags; DB layer is unaffected since pgx binds by
positional params, not struct-tag reflection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/store/tasks.go

@@ -3,12 +3,12 @@ package store
 import "context"
 
 type Task struct {
-	ID            int64
-	Name          string
-	SrcEndpointID int64
-	DstEndpointID int64
-	Status        string
-	FolderMapping map[string]string
+	ID            int64             `json:"id"`
+	Name          string            `json:"name"`
+	SrcEndpointID int64             `json:"src_endpoint_id"`
+	DstEndpointID int64             `json:"dst_endpoint_id"`
+	Status        string            `json:"status"`
+	FolderMapping map[string]string `json:"folder_mapping"`
 }
 
 func (s *Store) CreateTask(ctx context.Context, t Task) (int64, error) {