feat(server,webapp): retract delivered feedback and let developers dispute it in context

[FelixTJDietrich]

Outcome

Feedback that turns out to be wrong can be withdrawn by a person, and the developer who received it has a
way to say so from where they are reading it.

Problem

Hephaestus will be confidently wrong in public on someone's pull request. That is not a risk to mitigate;
it is a certainty to design for, and the first occurrence sets how much the team trusts everything after
it.

Today there is no product answer:

• No retraction. There is no retract API and no retracted state. The pieces exist —
  FeedbackPlacement stores the posted comment reference, and the provider channels can edit and minimise
  a comment — but they only fire from a subsequent review run. The only documented path is a manual
  operator procedure in the runbook (docs: production operations runbook #1371): find the delivery in the ledger, delete the comment through
  the provider API, mark the entry.
• No reachable dispute. The reaction path is server-complete, including recurrence suppression that
  stops a disputed observation being raised again, and it has zero call sites in the webapp. feat(webapp): developer reflection dashboard (per-practice observations and feedback) #1340
  carries an in-app version as one clause of a whole dashboard; a developer reading a wrong comment on a
  pull request has no path at all.

What

• A retract action a workspace admin can take on a delivered unit, driving the stored comment
  reference through the existing edit/minimise channels, with a retracted state in the ledger so the
  history stays honest. Correct in place rather than deleting: anyone who already read it should see the
  correction too.
• An in-context dispute affordance on delivered feedback, so the developer can mark it wrong where
  they are reading it, not only inside the web app.
• Un-gate the existing suppression so a disputed observation is not raised again on the same work.
• Count disputes per practice, feeding the practice-health signals (feat(server,webapp): admin per-practice observation and feedback analytics #1339).

Acceptance criteria

• An admin can retract a delivered comment; the external comment is corrected in place and the ledger
  records who retracted it and why.
• A developer can dispute delivered feedback from the surface it was delivered on.
• A disputed observation is not raised again for the same work.
• Retraction and dispute are both audited.

Out of scope

• Automatically retracting on a later run's disagreement. A retraction is a human act.
• Editing the wording of delivered feedback. This withdraws it; it does not rewrite it.