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/api/errors.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from typing import NoReturn
 
 from fastapi import FastAPI, HTTPException, Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 from starlette.exceptions import HTTPException as StarletteHTTPException
 
@@ -29,7 +30,23 @@ async def _handler(request: Request, exc: StarletteHTTPException) -> JSONRespons
     return JSONResponse(status_code=exc.status_code, content=body, headers=headers)
 
 
+async def _validation_handler(request: Request, exc: RequestValidationError) -> JSONResponse:
+    """A malformed request, in the usual error shape. Names the first field at
+    fault and what is wrong with it — enough to show the user — but not the
+    whole request model the framework's own answer would spell out."""
+    errors = exc.errors()
+    first = errors[0] if errors else {}
+    loc = [str(p) for p in first.get("loc", ()) if p not in ("body", "query", "path")]
+    field = loc[-1] if loc else ""
+    reason = str(first.get("msg", "invalid value"))
+    message = f"{field}: {reason}" if field else reason
+    return JSONResponse(
+        status_code=422, content={"error": {"code": "validation_error", "message": message}}
+    )
+
+
 def register_error_handlers(app: FastAPI) -> None:
     """Install the error-shape handler for all HTTP exceptions."""
     app.add_exception_handler(StarletteHTTPException, _handler)  # type: ignore[arg-type]
     app.add_exception_handler(HTTPException, _handler)  # type: ignore[arg-type]
+    app.add_exception_handler(RequestValidationError, _validation_handler)  # type: ignore[arg-type]