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/app/config.py

@@ -71,6 +71,9 @@ class Settings(BaseSettings):
     # Spelling and grammar checking in the compose window (LanguageTool server,
     # e.g. http://languagetool:8010). Empty: the check is off for everyone.
     languagetool_url: str = Field(default="", alias="LANGUAGETOOL_URL")
+    # New passwords are checked against known data breaches (Have I Been Pwned's
+    # k-anonymity range API: only 5 hex digits of a SHA-1 leave the server).
+    password_breach_check: bool = Field(default=True, alias="PASSWORD_BREACH_CHECK")
     # Auto-purge Trash/Spam messages older than this many days (0 disables).
     trash_retention_days: int = Field(default=30, alias="TRASH_RETENTION_DAYS")