Add stable IDs to webhook mappings

[chrysb]

Summary

• write a stable id on every webhook mapping created or updated by AlphaClaw
• backfill every missing mapping ID during onboarded startup, before the gateway launches
• derive readable IDs from path, name, or source, with deterministic mapping-N fallback IDs
• avoid collisions with existing and newly generated IDs while preserving all existing IDs
• cover Gmail-style legacy mappings and OAuth-enabled webhook creation

OpenClaw treats mapping IDs as optional and synthesizes positional IDs when absent. This migration makes those identities explicit and stable across reordering, improving diagnostics, auditing, and compatibility for existing user configurations. It is idempotent and requires no manual config edits.

Verification

• Node.js 22.22.3 full suite on the initial implementation: 102 files, 719 tests passed
• targeted webhook, OAuth callback, and startup tests pass with collision and fallback coverage
• git diff --check
• localhost gateway restarted healthy on OpenClaw 2026.7.1