Add a topic-centric view comparing MKs and parties on one issue

[yav02]

The UI only offers a per-MK and per-party grid (ui/src/App.tsx, PartyBoard, MKCard). There is no way to pick an issue and see where everyone stands on it — neither individual MKs side by side, nor parties as a whole. That is the other half of the product idea, and it is the view most readers actually want going into an election.

What to build

A view switcher in ui/src/components/ControlsHeader.tsx toggling between:

• תצוגת חברי כנסת / מפלגות — the current grid
• תצוגת נושאים / מטריצת עמדות — the new topic-centric view

A per-issue matrix. With an issue selected, group MKs into stance tiers — תמיכה / ודאות גבוהה, כיסוי חלקי / ודאות בינונית, ניטרלי, אין התייחסות — and show stated positions against actual voting records for that issue across parties.

A party-level aggregate. Alongside the individual members, show where each party stands as a whole on the selected issue, so the view answers "where does this party stand" and not only "where does this MK stand".

A backend endpoint. Something like /api/issues/{issue_key}/mks or /api/issues/matrix in src/mk_tracking/ui_app/api.py and repositories/bigquery_repository.py, so the view can be built from one request rather than assembled client-side.

A prototype already exists

The feature/per-party-stats branch carries a working prototype of the party-aggregate half: ui/src/components/PartyScoreAxis.tsx plots each party's members along a 1–5 rating axis for the selected issue, with the backend exposing a new ratings field per MK. It builds and runs, but the layout is unfinished and it has no test coverage. It is a starting point, not a finished feature.

Questions to settle before finishing it

• How to aggregate a party. A mean hides disagreement, which is often the interesting part. A distribution or range may represent a party more honestly than a single number.
• How to handle MKs with no evidence. They must not be folded into an average as though they were neutral. See the evidence rules in ui/CONTRIBUTING.md: a missing opinion is none, never neutral, and absence of evidence is not evidence of a moderate position.
• Minimum coverage. A party where two of thirty members have any evidence should probably show no aggregate at all.

Files likely involved

• ui/src/App.tsx
• ui/src/components/ControlsHeader.tsx
• a new ui/src/components/IssueBoard.tsx
• ui/src/components/PartyScoreAxis.tsx (on the prototype branch)
• src/mk_tracking/ui_app/api.py
• src/mk_tracking/ui_app/repositories/bigquery_repository.py

Note that the voting-record half depends on bills and votes reaching the summaries at all — see the issue on using bill votes and sponsorship in MK summaries.