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>

migrations/versions/943dbd74d983_add_token_version_to_users.py

@@ -0,0 +1,40 @@
+"""add token_version to users
+
+Revision ID: 943dbd74d983
+Revises: 861e76aaf34c
+Create Date: 2026-07-27
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '943dbd74d983'
+down_revision: Union[str, Sequence[str], None] = '861e76aaf34c'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # server_default backfills every existing user to 0 (their current sessions
+    # stay valid, since 0 also matches what already-issued tokens carry
+    # implicitly — see the "sub"-only tokens issued before this migration);
+    # dropped right after so new rows go through the ORM default instead of a
+    # stale constant.
+    op.add_column(
+        'users', sa.Column('token_version', sa.Integer(), nullable=False, server_default='0')
+    )
+    with op.batch_alter_table('users') as batch_op:
+        batch_op.alter_column('token_version', server_default=None)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('users', 'token_version')
+    # ### end Alembic commands ###