COE: Revise a COE amount same day or cancel

As an _Institution User_, _I want_ to revise a COE amount same day or cancel, _so that_, if I make a mistake, I can quickly correct it.

**Acceptance Criteria**
- [ ] Revise COE amount on the same day (TBD)
- [ ] Cancel a confirmed COE (TBD)

Ref. JIRA SBST-1922 (point 2) comment from PSI:
Being able to revise a COE amount same day would be huge, or to even be able to cancel the action.

**Additional Context**
- Accidentally confirming a different student's COE

No files, tests, or entry points are named. First clarify the two TBD acceptance criteria: the rules for revising a confirmed COE amount on the same day and for cancelling a confirmed COE, including the accidental-student case; done means both behaviors are explicitly defined and verifiable.

Written by the indexing model from the issue text.