fix: avoid races during interactive drive acceptance

[shepilov]

What happend:

• alice@twake.app created the shared drive invitation, at that point, the recipient's Cozy URL(bob@twake.app) was not yet registered on the owner side, so the system only sent the discovery email link
• because there was no recipient-side sharing request yet, trusted-domain auto-accept job had nothing to accept yet
• bob@twake.app clicked the discovery link from the email
• the discovery flow created the recipient-side sharing request with PUT /sharings/:id on the recipient Cozy, as soon as that request arrived, the recipient recognized the sender as trusted (linagora.com) and enqueued drive auto-accept.
• the browser then continued into the normal manual acceptance route: GET /auth/authorize/sharing?....
• both paths then touched the same sharing on the recipient side
• the sharing was actually accepted, and the owner side received a successful /answer, but the browser/manual path hit a stale CouchDB revision conflict and returned a 409 page.

Changes

• mark discovery/authorize acceptance as interactive(without auto-acceptance flow)
• make SendAnswer idempotent on conflicts

[shepilov]

also, the next step will be this issue #4721