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_otc_vertical.py

@@ -347,3 +347,53 @@ def test_otc_re_request_invalidates_prior_unused_code(app_with_fake_gitea, monke
         # The new code still works.
         r = client.post("/auth/otc/verify", json={"email": "alice@example.com", "code": second})
         assert r.status_code == 200
+
+
+# ---------------------------------------------------------------------------
+# v0.18.0: envelope headers — Slice 2
+#
+# OTC mail goes through `build_envelope` and MUST land Date,
+# Message-ID, and Auto-Submitted but MUST NOT carry a
+# List-Unsubscribe header (the recipient explicitly requested the
+# code; advertising a list semantic would be wrong).
+# ---------------------------------------------------------------------------
+
+
+def _last_otc_envelope():
+    from app import email as email_mod
+    otc = [e for e in email_mod.sent_envelopes() if e.get("kind") == "otc"]
+    assert otc, "no OTC envelope in the buffer"
+    return otc[-1]
+
+
+def test_otc_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:
+        _reset_outbound()
+        client.post("/auth/otc/request", json={"email": "headers@example.com"})
+        msg = _last_otc_envelope()["message"]
+        # Date is RFC 5322 parseable.
+        assert parsedate_to_datetime(msg["Date"]) is not None
+        # Message-ID is bracketed and carries the From-address @-domain.
+        mid = msg["Message-ID"]
+        assert mid.startswith("<") and mid.endswith(">")
+        # Auto-Submitted prevents auto-responder loops.
+        assert msg["Auto-Submitted"] == "auto-generated"
+
+
+def test_otc_envelope_has_no_list_unsubscribe(app_with_fake_gitea):
+    """The recipient explicitly typed their email and asked for a
+    code; the framework MUST NOT advertise a list semantic on this
+    mail. Per the v0.18.0 proposal's tradeoff discussion."""
+    from fastapi.testclient import TestClient
+
+    app, _fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        _reset_outbound()
+        client.post("/auth/otc/request", json={"email": "headers@example.com"})
+        msg = _last_otc_envelope()["message"]
+        assert msg["List-Unsubscribe"] is None
+        assert msg["List-Unsubscribe-Post"] is None