Invalidate existing sessions on password change/reset (B-34)

Neither self-service password change nor the admin reset invalidated
already-issued JWTs — a 24h-lifetime token stayed valid regardless, so
a stolen token (or an attacker who already had the old password) kept
working past a password change meant to lock them out. The admin reset
exists precisely for the "account compromised" case and didn't evict
the attacker at all.

Add User.token_version (migration 943dbd74d983), embedded in every JWT
as a "tv" claim and checked against the DB on every request in
get_current_user/get_optional_user; a mismatch reads as session_expired.
Both change-password and the admin reset bump it. change-password hands
back a freshly minted token so the caller's own session keeps working
instead of being logged out by its own request; the admin reset does
not, since that session isn't the one making the call.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/routes/admin.py

@@ -215,6 +215,11 @@ async def reset_user_password(
 
     new_password = secrets.token_urlsafe(12)
     user.password_hash = hash_password(new_password)
+    # B-34: this endpoint exists precisely for the "account compromised" case —
+    # without bumping token_version, whoever was already logged in (the
+    # attacker, if that's who prompted the reset) stayed logged in on their
+    # existing token until it naturally expired, unaffected by the reset.
+    user.token_version += 1
     await write_audit_log(session, "admin_password_reset", {"user_id": user_id}, user_id=user_id)
     await session.commit()
     return AdminPasswordResetResponse(username=user.username, new_password=new_password)

app/api/routes/users.py

@@ -5,7 +5,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.errors import http_error
 from app.auth.dependencies import get_current_user
-from app.auth.security import MIN_PASSWORD_LENGTH, hash_password, verify_password
+from app.auth.security import MIN_PASSWORD_LENGTH, create_access_token, hash_password, verify_password
 from app.db.models import Round, RoundParticipant, User
 from app.db.session import get_session
 from app.wallet.balance import compute_pending_balance
@@ -45,12 +45,16 @@ class ChangePasswordRequest(BaseModel):
     new_password: str
 
 
-@router.post("/me/change-password", status_code=status.HTTP_204_NO_CONTENT)
+class ChangePasswordResponse(BaseModel):
+    access_token: str
+
+
+@router.post("/me/change-password", response_model=ChangePasswordResponse)
 async def change_password(
     body: ChangePasswordRequest,
     user: User = Depends(get_current_user),
     session: AsyncSession = Depends(get_session),
-) -> None:
+) -> ChangePasswordResponse:
     """Self-service password change — requires the current password, unlike the
     admin-only /admin/users/{id}/reset-password (which is for a user who's
     actually locked out and can't provide it)."""
@@ -67,7 +71,15 @@ async def change_password(
         )
 
     user.password_hash = hash_password(body.new_password)
+    # B-34: bumping token_version invalidates every token issued before this
+    # point — including this very request's own bearer token, and any an
+    # attacker who knew the old password might be holding. A fresh token is
+    # handed back so *this* session keeps working without forcing a re-login;
+    # every other open session (this user's other devices, or an attacker's)
+    # gets "session_expired" on its next request.
+    user.token_version += 1
     await session.commit()
+    return ChangePasswordResponse(access_token=create_access_token(user.id, user.token_version))
 
 
 class LastRoundResultResponse(BaseModel):