v0.18.0 Slice 4: outbound_emails audit table + admin endpoint

Per the v0.18.0 email + webhook hygiene proposal §3:

  * `backend/migrations/020_outbound_emails.sql` — new table
    capturing every send attempt: to_address, from_address,
    subject, kind, sent_at, status ('sent' | 'failed' | 'deferred'
    | 'bounced'), error, notification_id (FK), message_id (for
    Slice 5 bounce correlation).
  * `email.record_outbound()` — best-effort write helper every
    send path calls. Status='sent' on SMTP success, 'failed' on
    exception (with class + message in `error`), 'deferred' on
    the dev-fallback path where SMTP_HOST is unset (the send
    didn't happen but the row records the attempt).
  * `email._deliver` (watcher notifications + bundles), `digest.py`,
    `email_otc.py`, `email_invite.py` — every send path now records.
  * `GET /api/admin/outbound-emails` — admin-only listing,
    filterable by kind / status / to_address. Answers "did this
    person ever get their invite?" without grepping VM logs. No
    admin UI in v0.18.0; operator queries via curl + jq for now.

7 new integration tests covering: OTC / invite / notification all
write rows with matching Message-ID; admin endpoint lists,
filters by kind, filters by to_address (case-insensitive),
refuses non-admins.

Full suite: 291 passed.

backend/app/api_admin.py

@@ -672,6 +672,73 @@ def make_router(config: Config) -> APIRouter:
             "has_more": len(rows) == limit,
         }
 
+    @router.get("/api/admin/outbound-emails")
+    async def list_outbound_emails(
+        request: Request,
+        kind: str | None = None,
+        status: str | None = None,
+        to_address: str | None = None,
+        limit: int = Query(default=100, ge=1, le=500),
+        before_id: int | None = None,
+    ) -> dict[str, Any]:
+        """v0.18.0 Slice 4: read-only inspection of the
+        `outbound_emails` audit table.
+
+        Answers questions like "did this person ever get their
+        invite?" without grepping VM logs. Filterable by kind
+        ('otc' | 'invite' | 'notification' | 'bundle' | 'digest'),
+        status ('sent' | 'failed' | 'deferred' | 'bounced'), and
+        to_address; the latter is exact-match because the audit
+        question is usually "the specific person who said they
+        didn't receive it." Per the proposal, no admin UI ships
+        with v0.18.0 — operator queries via curl + jq for now.
+        """
+        auth.require_admin(request)
+        clauses: list[str] = []
+        args: list[Any] = []
+        if kind:
+            clauses.append("kind = ?")
+            args.append(kind)
+        if status:
+            clauses.append("status = ?")
+            args.append(status)
+        if to_address:
+            clauses.append("LOWER(to_address) = LOWER(?)")
+            args.append(to_address)
+        if before_id is not None:
+            clauses.append("id < ?")
+            args.append(before_id)
+        where = ("WHERE " + " AND ".join(clauses)) if clauses else ""
+        rows = db.conn().execute(
+            f"""
+            SELECT id, to_address, from_address, subject, kind, sent_at,
+                   status, error, notification_id, message_id
+              FROM outbound_emails
+             {where}
+             ORDER BY id DESC
+             LIMIT ?
+            """,
+            (*args, limit),
+        ).fetchall()
+        return {
+            "items": [
+                {
+                    "id": r["id"],
+                    "to_address": r["to_address"],
+                    "from_address": r["from_address"],
+                    "subject": r["subject"],
+                    "kind": r["kind"],
+                    "sent_at": r["sent_at"],
+                    "status": r["status"],
+                    "error": r["error"],
+                    "notification_id": r["notification_id"],
+                    "message_id": r["message_id"],
+                }
+                for r in rows
+            ],
+            "has_more": len(rows) == limit,
+        }
+
     @router.get("/api/admin/permission-events")
     async def list_permission_events(
         request: Request,