As an instruktør I want to mark a session private so it is excluded from the shared class history

[ChristopherRotnes]

Summary

Sharing is opt-out by default (#136 decision). This issue adds a `visibility` flag on `sessions` so an instruktør can mark a specific session as private, removing it from the joint-class history view (issue #138) without deleting it.

Priority

Medium

Data model

alter table sessions add column visibility text not null default 'shared'
  check (visibility in ('shared', 'private'));

The default `'shared'` preserves existing behaviour for all current rows. A `'private'` row is excluded from cross-instruktør queries but remains fully visible to its owner.

UI spec (Carbon g100)

• In History edit mode: add a Carbon `Toggle` labelled "Del med andre instruktører" (default on) below the session date picker
• When toggled off, set `visibility = 'private'` on save
• In the joint-class history view (As an instruktør I want to see joint session history for a class slot so I can avoid repeating what a colleague already trained #138): exclude rows where `visibility = 'private'`

Acceptance criteria

• `visibility` column added to `sessions` via Supabase migration with default `'shared'`
• `updateSession` in `db.js` accepts and persists the `visibility` value
• History edit mode exposes the toggle (Carbon `Toggle`, sentence-case label)
• Joint-class history query (As an instruktør I want to see joint session history for a class slot so I can avoid repeating what a colleague already trained #138) filters out private sessions
• RLS: private sessions remain readable by the owner; the cross-gym policy must exclude them
• Verified by developer on dev/qa/prod before closing

Out of scope

• Per-exercise privacy
• Retroactive bulk opt-out

[ChristopherRotnes]

Delivered in PR #146 — sessions.visibility column (shared/private), edit-mode Toggle auto-saves instantly, cross-gym RLS excludes private sessions.

[ChristopherRotnes]

We tried this in a branch but it didnt work well and made the app worse. So the entire feature is removed.