Classroom join codes

AGENTS.md

@@ -77,6 +77,17 @@ from `showDriverStation` to make that possible. See
 `docs/decisions/041-project-preview.md` and
 [`docs/using-coderunner.md`](./docs/using-coderunner.md).
 
+**Classroom join codes (post-V2):** admins start a short-lived classroom
+(Admin → Classrooms) with a 6-digit code; students join at `/join` with the
+code and their name and become guest users (`user.classroomId` set, role
+`student`, no allowlist). A custom Better Auth plugin
+(`auth/classroom-plugin.ts`) mints the session; `app/classroom-routes.ts`
+rate-limits failed attempts and retires the browser's previous guest
+session. Guest sessions die with the classroom (checked in
+`getSessionFromRequest`), guests have a **Leave** button that stops their
+container, and `ClassroomSweeper` deletes guests of ended/expired
+classrooms. See `docs/decisions/043-classroom-join-codes.md`.
+
 **Containerized control plane (post-V2):** the control plane ships as a Docker
 image (`containers/control/Dockerfile` → `ghcr.io/mathewdunne/coderunner-control`)
 and is deployed with docker compose (`docker-compose.yml` base +
@@ -134,7 +145,7 @@ arch-independent). See `docs/decisions/035-multi-arch-images-and-workflow-split.
 ## Key References
 
 - `docs/` + `website/` — docs site content and Docusaurus config; published at `https://mathewdunne.github.io/CodeRunner/`; run `bun run docs:dev` to browse locally, `bun run docs:build` to build.
-- `docs/decisions/` — all architecture decision logs (011–041 active; 001–010 archived under `docs/decisions/archive/`).
+- `docs/decisions/` — all architecture decision logs (011–043 active; 001–010 archived under `docs/decisions/archive/`).
 - Pinned AdvantageScope submodule: `vendor/AdvantageScope` at tag `v26.0.2`.
 
 ## Commands

apps/control/migrations/012_classrooms.sql

@@ -0,0 +1,17 @@
+-- Classroom join codes (decision 043). Guest users point at a classroom via
+-- the Better Auth user.classroomId column, which Better Auth itself adds
+-- (declared as an additionalField) — the user table does not exist yet when
+-- these migrations run.
+CREATE TABLE classrooms (
+  id TEXT PRIMARY KEY,
+  code TEXT NOT NULL,
+  created_by TEXT NOT NULL,
+  created_at TEXT NOT NULL,
+  expires_at TEXT NOT NULL,
+  ended_at TEXT,
+  cleaned_at TEXT
+);
+
+-- Codes are unique only among live classrooms; "live" depends on now(), so
+-- uniqueness is enforced at creation time rather than by a constraint.
+CREATE INDEX idx_classrooms_code ON classrooms(code);