feat(google): group→Google-Group binding model and CRUD API

[BK1031]

PR 2 of the Sentinel-group → Google-Group sync feature. Adds the mapping table and its admin API. Still no Google API calls.

What's here

• GroupGoogleBinding model (group_google_binding): 1:1 mapping of a Sentinel group_id → google_group_email. Both columns unique (one Sentinel group projects onto one Google Group, and each Google Group is driven by a single Sentinel group). AutoMigrated.
• Service layer: GetAllGoogleBindings, GetGoogleBindingForGroup, CreateGoogleBinding, DeleteGoogleBinding (delete scoped to group_id + id, mirroring the discord binding).
• sentinel:all-gated CRUD endpoints:
  • GET /google/group-bindings (optional ?group_id= filter)
  • POST /google/group-bindings (validates email format)
  • DELETE /google/group-bindings/:bindingID?group_id=

Design notes

• 1:1 chosen over the discord-style multi-binding/OR model — outbound projection has no need for it, and uniqueness keeps the reconcile trivial.
• Email is stored (not Google's internal group id) since it's what admins know and the Directory API accepts it as the groupKey.
• No group-existence check against core on create (matches the discord binding); reconcile is the source of truth for correctness.

Not in this PR

• Google Admin SDK Directory client + reconcile engine + cron (PR 3)
• Infra: k8s manifest, deploy.yml, kerbecs gateway route (PR 4)

Verification

• go build ./... + go vet ./... pass