invites: an admin lets one person register while registration stays closed

Until now adding a second person meant re-opening registration to the
whole internet while they signed up (#2939 §1). An admin now makes an
invite in Settings: a link that works once, expires (7 days by default,
1 to 30), and can be pinned to one email address. Only the token's hash
is stored, so the link is shown once.

POST /api/auth/register takes `invite`. Redemption is one conditional
UPDATE inside the transaction that creates the account, so two people
racing one link can't both get in, and a taken email leaves the invite
unused. Every refusal says "invalid or expired invite". The register
page reads ?invite= and opens even while registration is closed.

Refs #5172

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

alembic/versions/0032_invites.py

@@ -0,0 +1,42 @@
+"""invites: single-use, expiring registration links an admin issues
+
+Revision ID: 0032
+Revises: 0031
+Create Date: 2026-10-07
+
+Until now the only way to add a second person was to re-open registration to the
+whole internet while they signed up (#2939 §1). An invite lets one person register
+while registration stays closed. Only the token's SHA-256 hash is stored.
+
+## Downgrade
+
+Drops the table. Accounts created through invites are untouched; the record of who
+invited them goes with it.
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import CITEXT, UUID
+
+revision = "0032"
+down_revision = "0031"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "invites",
+        sa.Column("id", UUID(as_uuid=True), primary_key=True),
+        sa.Column("token_hash", sa.Text(), nullable=False, unique=True),
+        sa.Column("created_by", UUID(as_uuid=True), sa.ForeignKey("users.id", ondelete="SET NULL"), nullable=True),
+        sa.Column("email", CITEXT(), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.func.now()),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("redeemed_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("redeemed_by", UUID(as_uuid=True), sa.ForeignKey("users.id", ondelete="SET NULL"), nullable=True),
+        sa.Column("revoked_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("invites")