convert pdf to md, add OVERVIEW and docs

worker-toolkit-stocks-in-the-future/repo/docs/map/objects/identity/teacher.md

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+---
+type: object
+cluster: identity
+universe: live
+status: verified
+entity: app/models/teacher.rb
+---
+
+# Teacher
+
+A `User` with `type: "Teacher"` — runs classrooms and gradebooks. Owns no portfolio and
+cannot trade.
+
+Verified 2026-08-16 against commit `63732df`.
+
+## Why this shape
+
+Login is by `username` app-wide, but teachers think in email addresses. Rather than
+splitting the auth key, `Teacher` **copies email into username** on every validation
+(`app/models/teacher.rb:9,17-19`). So a teacher's username is their email, kept in sync
+automatically — change the email and the login changes with it. This is the exact inverse
+of [student](student.md), whose username is assigned and whose email is usually `nil`.
+
+`attr_accessor :school_id` (`:4`) is a form-only field. It is **not a column and not
+persisted** — a teacher reaches a school only through classrooms.
+
+## Shape
+
+- STI subclass of [user](user.md); no table of its own
+- `has_many :teacher_classrooms`, `has_many :classrooms, through:` (`:6-7`)
+- `before_validation :sync_username_from_email` (`:9`)
+- `display_name` prefers `name`, then the email local-part (`:11-13`)
+- Join table `teacher_classrooms` — unique on `[teacher_id, classroom_id]`
+  (`db/schema.rb:368`). No behaviour of its own, so it has no card.
+
+## Connected to
+
+- **owns:** [classroom](../org/classroom.md) (through `teacher_classrooms`)
+- **owned-by:** —
+- **joins:** `teacher_classrooms`
+- **looks-like-but-is-not:** a teacher is not an admin. Admin is a boolean on `users`;
+  `teacher_or_admin?` (`app/models/user.rb:53-55`) exists precisely because the two are
+  independent and often both true.
+
+## If you change this
+
+- **Hits:** sign-in for every teacher if you touch `sync_username_from_email` — it
+  rewrites `username`, the auth key; `Order.for_teacher`
+  (`app/models/order.rb:40-42`), which scopes orders through
+  `users.classroom_id`, **not** through `teacher_classrooms`;
+  `Admin::Teachers::DeactivationsController` / `ReactivationsController`.
+- **Does not hit:** [portfolio](../money/portfolio.md). `Portfolio` validates that its
+  user is a student (`app/models/portfolio.rb:102-104`), so no teacher change can create
+  or affect one.
+
+## Surfaces
+
+| Surface | Role |
+|---|---|
+| `Admin::TeachersController` | admin CRUD |
+| `Admin::Teachers::DeactivationsController` / `ReactivationsController` | discard / restore |
+| `ClassroomsController`, `GradeBooksController` | authorizes as teacher |
+
+## See
+
+- Source: `app/models/teacher.rb`
+- Base class: [user](user.md)