Add endpoint for creating a policy acceptance for a specific user

[dati18]

Summary

• Add authenticated endpoint to record policy acceptances for the current user, with strict validation and all-or-nothing behavior for invalid policy IDs.

Behavior

• Uses authenticated user from session context (no user_id in payload)
• Ignores already-accepted policies (idempotent writes via firstOrCreate)
• If any provided policy ID does not exist, returns 400 with missing_policy_ids
• No acceptances are created when invalid policy IDs are present
• Returns 422 for invalid payload (missing policy_ids, non-array, non-integer items)

Bug: T431674

[tarrow]

Only blocker from my side is around confirming we don't update accepted_at time on each request.

[tarrow]

Thanks for the final change. I'm now happy with this.