Three coupled fixes from the first manual test pass:
1. Stale signed cookie no longer 500s. `rooms.me()` now raises KeyError
when the participant row is gone (the previous code deref'd None and
threw TypeError, caught by quiz.js's catch-all as 'link expired').
`/api/session/{sid}/me` translates KeyError into 401 + delete_cookie,
so the client falls back to the join form cleanly.
Returning a JSONResponse directly because `raise HTTPException`
discards Response.delete_cookie mutations (FastAPI middleware
composes a fresh response on exception).
2. Reset is now a soft restart from the student's perspective. Before
closing each student WS in `RoomManager.reset`, the server now sends
a `{"type": "session_reset"}` message. The student SPA tears down
local state and re-runs boot(); /me returns 401 (now that the
participant is gone) and the join form renders without the user
having to manually reload. The WS close handler suppresses its
"Disconnected" screen during a reset to avoid a flash.
3. "You" highlight on the student leaderboard is now matched by id, not
by name. `RoomManager.leaderboard()` accepts an optional
`you_student_id` and stamps `is_you: true` on the matching entry only.
No other students' ids leak over the wire (we still don't include
`student_id` in the public top5 payload). quiz.js's renderBoard
prefers `r.is_you` when any row is marked, falling back to name match
for backward compatibility.
41/41 tests pass. Two new tests cover (a) the 401 + cookie-clear path
after reset and (b) `is_you` marking only the requesting student.
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