feat(auth): reset a forgotten password from an emailed link (milestone 497)

Step #5429 "Reset by email: request, link and new-password page" of
milestone 497 "Passwords: change your own, and reset it by email".

- Sign-in page: "Forgot your password?" while a mail server is set.
- /forgot takes a username or email and answers the same either way; only
  an active local account with a real address gets mail. Requests count
  per address on their own throttle, so they never lock out sign-in.
- password_resets (migration 0039): a one-time key, stored as its sha256,
  working for an hour; asking again replaces it.
- /reset/<key>: the new password, then signed straight in; every other
  browser is signed out (sessions_since). Bad keys count as invite keys do.
- Email "[Steward] Reset your password", in the account's own zone.
- Audited: user.password_reset_requested, user.password_reset_by_email.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/migrations/versions/0039_password_resets.py

@@ -0,0 +1,33 @@
+"""Reset a local password from an emailed link (milestone 497): a one-time
+key, stored as its sha256, that works for an hour.
+
+Revision ID: 0039_password_resets
+Revises: 0038_user_sessions_since
+Create Date: 2026-10-09
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0039_password_resets"
+down_revision: Union[str, None] = "0038_user_sessions_since"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "password_resets",
+        sa.Column("id", sa.Integer, primary_key=True, autoincrement=True),
+        sa.Column("user_id", sa.String(36), sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("key_hash", sa.String(64), nullable=False, unique=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("used_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.create_index("ix_password_resets_user_id", "password_resets", ["user_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_password_resets_user_id", table_name="password_resets")
+    op.drop_table("password_resets")