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>

tests/unit/test_admin.py

@@ -205,6 +205,10 @@ async def test_admin_resets_user_password(client):
         assert refreshed.password_hash != old_hash
         assert verify_password(new_password, refreshed.password_hash)
         assert not verify_password("original-password", refreshed.password_hash)
+        # B-34: the reset must bump token_version so a session opened before
+        # the reset (e.g. an attacker who had the old password) is evicted
+        # immediately rather than staying valid until the JWT naturally expires.
+        assert refreshed.token_version == 1
 
 
 async def test_admin_reset_password_requires_token(client):