Send magic-link email in the background (instant request response)

The SMTP send (connect → TLS → login → handoff to the relay) ran synchronously
inside POST /api/auth/email/start, so the "Sending…" button waited the whole
handshake. Move it to a FastAPI BackgroundTask: the token is created + committed,
the request returns immediately, and the email sends off the request path. Reply
stays identical (no account enumeration). Tests pass (TestClient runs the task).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/api.py

@@ -22,7 +22,7 @@ from contextlib import contextmanager
 from datetime import datetime, timezone
 from pathlib import Path
 
-from fastapi import FastAPI, HTTPException, Query, Request, Response
+from fastapi import BackgroundTasks, FastAPI, HTTPException, Query, Request, Response
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel
@@ -74,6 +74,14 @@ def _current_user(conn: sqlite3.Connection, request: Request) -> sqlite3.Row | N
     return user
 
 
+def _send_link_safe(email: str, link: str) -> None:
+    """Send the magic link, swallowing failures (runs off the request path)."""
+    try:
+        email_send.send_magic_link(email, link)
+    except Exception:
+        pass  # don't crash the worker; never surfaced to the caller anyway
+
+
 def _set_session_cookie(response: Response, token: str) -> None:
     response.set_cookie(
         SESSION_COOKIE, token, max_age=SESSION_MAX_AGE,
@@ -285,10 +293,11 @@ def create_app() -> FastAPI:
     # --- Auth: passwordless magic link (Google added in Phase 2) ----------
 
     @app.post("/api/auth/email/start")
-    def auth_email_start(body: EmailStartRequest) -> dict:
+    def auth_email_start(body: EmailStartRequest, background_tasks: BackgroundTasks) -> dict:
         email = auth.normalize_email(body.email)
         if not _EMAIL_RE.match(email):
             raise HTTPException(status_code=422, detail="Please enter a valid email address.")
+        link = None
         with get_conn() as conn:
             # Light abuse guard: cap recent tokens per address (still reply OK).
             recent = conn.execute(
@@ -300,11 +309,10 @@ def create_app() -> FastAPI:
                 raw = auth.create_login_token(conn, email)
                 conn.commit()
                 link = f"{PUBLIC_BASE_URL}/auth/verify?token={raw}"
-                try:
-                    email_send.send_magic_link(email, link)
-                except Exception:
-                    pass  # never leak send failures or whether the address exists
-        # Always identical (no account enumeration).
+        # Hand the (slow) SMTP send to a background task so the request returns
+        # immediately. Reply is always identical (no account enumeration).
+        if link:
+            background_tasks.add_task(_send_link_safe, email, link)
         return {"ok": True}
 
     @app.post("/api/auth/email/verify", response_model=SessionOut)