Security hardening from the assessment; testing fixes

Security (ultimo-security-report.md):
- Security headers on every response, the app shell included: they lived at
  server level and nginx dropped them in any location with its own header.
  CSP, framing, nosniff, referrer, permissions (docker/nginx-security.conf).
- Plain HTTP forwarded by the reverse proxy is redirected to HTTPS; HSTS on
  HTTPS responses. server_tokens off.
- Cross-site state-changing /api requests refused (login CSRF), WebSocket too.
- Validation errors in the usual error shape, without the request model.
- New-password policy: max 128, common/own-address/repeated refused,
  breached-password check (HIBP k-anonymity, fails open).
- Message HTML: links keep only http(s)/mailto (sanitizer v6); the HTML
  endpoint carries its own no-script CSP.
- Outbound fetches pinned to the checked IP (DNS rebinding); external
  IMAP/SMTP servers re-checked before each connection; CGNAT and mapped
  addresses counted as non-public.

Fixes from testing:
- IDLE loop catches up before idling: mail that came while the worker was
  away no longer waits up to 10 minutes.
- Restore from Spam/Trash/Archive moves only the removed messages, not your
  reply in Sent.
- Narrow desktop list stacks sender/subject/snippet (subject was squeezed
  to nothing at the default width), with hover actions.
- Phone: wide emails scale to fit the screen.
- Singular forms ("1 message"); no zip button for a single attachment;
  app passwords no longer claim "never used" for mail apps.
- ruff format across the backend.

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

backend/tests/plugins/test_calendar_reminders.py

@@ -104,7 +104,8 @@ async def test_due_reminders_are_sent_once(ctx) -> None:  # type: ignore[no-unty
     assert await run(MON_9 - timedelta(minutes=60)) == 1  # the 1-hour reminder
     assert sent[-1]["title"] == "Dentist"
     assert sent[-1]["body"] == "In 1 hour, 09:00 · Main St 4"
-    assert sent[-1]["url"].startswith("/p/calendar?date=2031-06-02&event=") and sent[-1]["sticky"] is True
+    assert sent[-1]["url"].startswith("/p/calendar?date=2031-06-02&event=")
+    assert sent[-1]["sticky"] is True
     assert await run(MON_9 - timedelta(minutes=59)) == 0  # not again
     # The worker was down when the 10-minute reminder came due: sent late, once.
     assert await run(MON_9 - timedelta(minutes=4)) == 1