* feat: afterMsgSaveDB webhook
fix(config): add default webhook event filters
fix(core): handle group mentions and webhook enqueue errors
Use group IDs when filtering group-message attention callbacks, and report failures when asynchronous webhook tasks cannot be queued or posted.
feat(webhook): add standalone mock webhook server CLI
test(webhook): add mock server and test suite for all callback commands
feat(msgtransfer): trigger afterMsgSaveDB webhook when message saved to DB
feat(config): add afterMsgSaveDB webhook configuration
feat(callbackstruct): add CallbackAfterMsgSaveDB command and request structs
* refactor(core): simplify asynchronous webhook enqueueing
