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/scripts/backfill_delivery_id.py

@@ -16,9 +16,7 @@ from app.models import Message
 
 async def main() -> None:
     async with session_scope() as session:
-        rows = list(
-            await session.scalars(select(Message).where(Message.delivery_id.is_(None)))
-        )
+        rows = list(await session.scalars(select(Message).where(Message.delivery_id.is_(None))))
         fixed = 0
         for m in rows:
             try:

backend/scripts/reindex_address_parts.py

@@ -42,9 +42,7 @@ async def main() -> None:
                 update(Message)
                 .where(Message.id == msg.id)
                 .values(
-                    body_indexed=Message.body_indexed.op("||")(
-                        func.to_tsvector(_TS_CONFIG, blob)
-                    )
+                    body_indexed=Message.body_indexed.op("||")(func.to_tsvector(_TS_CONFIG, blob))
                 )
             )
             updated += 1