v0.18.0 Slice 2: migrate send paths to build_envelope + POST unsubscribe

Five send sites now construct their envelopes through
`email_envelope.build_envelope` instead of building `EmailMessage`
ad hoc:

  * `email_otc.py` — OTC mail (no List-Unsubscribe per the
    proposal's tradeoff: the recipient explicitly requested the
    code, so a list semantic would be wrong).
  * `email_invite.py` — admin/per-RFC invite mail (mailto:-only
    List-Unsubscribe — the invitee isn't a user yet, so no
    per-user opt-out URL exists).
  * `email._deliver` — watcher notifications (full one-click
    unsubscribe: mailto + signed URL + List-Unsubscribe-Post per
    RFC 8058, required by Gmail/Yahoo).
  * `email._send_bundle` — the "while you were away" bundle,
    one-click to the new `all` synthetic category (which lands
    `email_opt_out_all = 1` because the bundle spans multiple
    per-category flags).
  * `digest.py` — same as the bundle: bulk-adjacent, one-click to
    `all`.

`api_notifications.py` gains the POST `/api/email/unsubscribe`
endpoint (the matching receiver for `List-Unsubscribe-Post:
List-Unsubscribe=One-Click`) and accepts the `all` category in
both GET and POST handlers.

`EmailConfig` adds `unsubscribe_mailto` (env: `EMAIL_UNSUBSCRIBE_MAILTO`,
default = `EMAIL_FROM`) so deployments can route unsubscribe
courtesy mail to a humans-monitored mailbox distinct from the
no-reply sender.

The `_SENT` test buffer now also carries `envelope["message"]`
(the `EmailMessage` itself) so new tests can assert on headers
directly. Legacy `to`/`from`/`subject`/`body` keys remain for
backward compatibility.

10 new integration tests across test_otc_vertical /
test_admin_create_user_invite_vertical / test_notifications_vertical
covering: OTC has no List-Unsubscribe; invite has mailto: only;
notification has full one-click; POST one-click flips the
category; `all` category sets global opt-out via both GET and
POST.

Full suite: 277 passed.

backend/tests/test_admin_create_user_invite_vertical.py

@@ -647,3 +647,82 @@ def test_pending_invites_listing_admin_only(app_with_fake_gitea):
         )
         r = client.get("/api/admin/users/invites")
         assert r.status_code == 403
+
+
+# ---------------------------------------------------------------------------
+# v0.18.0: invite-envelope header shape — Slice 2
+#
+# Invite mail goes through `build_envelope` and MUST land Date,
+# Message-ID, Auto-Submitted, AND a `List-Unsubscribe: <mailto:…>`
+# (no URL — the invitee isn't a user yet, so no per-user opt-out
+# row exists). The mailto: target is the operator's `EMAIL_FROM`
+# by default; the operator can override via `EMAIL_UNSUBSCRIBE_MAILTO`.
+# ---------------------------------------------------------------------------
+
+
+def _provision_admin_and_send_invite(client, app_with_fake_gitea_fixture, *, to: str = "headers@ex.co"):
+    provision_user_row(user_id=400, login="adminH", role="admin")
+    sign_in_as(
+        client, user_id=400, gitea_login="adminH",
+        display_name="Admin H", role="admin",
+        email="adminh@test",
+    )
+    _reset_outbound()
+    r = client.post(
+        "/api/admin/users",
+        json={
+            "email": to,
+            "first_name": "Header",
+            "last_name": "Test",
+            "role": "contributor",
+            "custom_message": "",
+        },
+    )
+    assert r.status_code == 200, r.text
+    return _outbound_invite_envelopes(to)[-1]
+
+
+def test_invite_envelope_sets_date_messageid_autosubmitted(app_with_fake_gitea):
+    from fastapi.testclient import TestClient
+    from email.utils import parsedate_to_datetime
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        env = _provision_admin_and_send_invite(client, (app, _fake))
+        msg = env["message"]
+        assert parsedate_to_datetime(msg["Date"]) is not None
+        assert msg["Message-ID"].startswith("<") and msg["Message-ID"].endswith(">")
+        assert msg["Auto-Submitted"] == "auto-generated"
+
+
+def test_invite_envelope_has_mailto_list_unsubscribe_only(app_with_fake_gitea):
+    """The invitee isn't a user yet — no per-user opt-out URL is
+    available. The `List-Unsubscribe` MUST be a mailto: form, and
+    the `List-Unsubscribe-Post` header MUST be absent (the
+    one-click semantic requires a URL the MUA can POST to)."""
+    from fastapi.testclient import TestClient
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        env = _provision_admin_and_send_invite(client, (app, _fake))
+        msg = env["message"]
+        lu = msg["List-Unsubscribe"]
+        assert lu is not None and lu.startswith("<mailto:")
+        # No URL part — invite is mailto-only.
+        assert "https://" not in lu and "http://" not in lu
+        assert msg["List-Unsubscribe-Post"] is None
+
+
+def test_invite_envelope_respects_email_unsubscribe_mailto_override(app_with_fake_gitea, monkeypatch):
+    """When `EMAIL_UNSUBSCRIBE_MAILTO` is set, the mailto: target on
+    `List-Unsubscribe` honors it (lets a deployment route opt-outs
+    to a humans-monitored mailbox distinct from the no-reply
+    sender)."""
+    from fastapi.testclient import TestClient
+
+    monkeypatch.setenv("EMAIL_UNSUBSCRIBE_MAILTO", "ohm@wiggleverse.org?subject=remove")
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        env = _provision_admin_and_send_invite(client, (app, _fake))
+        msg = env["message"]
+        assert "ohm@wiggleverse.org?subject=remove" in msg["List-Unsubscribe"]