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/actions.py

@@ -125,9 +125,7 @@ async def mark_all_read(
         return {"changed": 0}
     messages = list(
         await db.scalars(
-            select(Message).where(
-                Message.thread_id.in_(thread_ids), Message.user_id == user.id
-            )
+            select(Message).where(Message.thread_id.in_(thread_ids), Message.user_id == user.id)
         )
     )
     add, remove = sanitize_flags(["\\Seen"]), sanitize_flags([])
@@ -137,9 +135,7 @@ async def mark_all_read(
     await _push_and_publish(user.id, changed, add, remove)
     bus = get_event_bus()
     for tid in thread_ids:
-        await bus.publish(
-            "thread.updated", {"user_id": str(user.id), "thread_id": str(tid)}
-        )
+        await bus.publish("thread.updated", {"user_id": str(user.id), "thread_id": str(tid)})
     return {"changed": len(changed)}
 
 

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]

backend/app/api/messages.py

@@ -88,6 +88,23 @@ def _wrap(fragment: str) -> str:
     return _DOC_TEMPLATE.format(body=fragment)
 
 
+# The sanitized message as a document of its own. The app shows it in a
+# sandboxed frame (frontend MessageBody), but this URL can also be opened
+# directly in a tab, on our own origin: there, this policy is what keeps any
+# script the sanitizer might ever miss from running — no scripts, no plugins,
+# no forms, a sandbox of its own origin, and nothing framing it elsewhere.
+_MESSAGE_CSP = (
+    "default-src 'none'; script-src 'none'; object-src 'none'; base-uri 'none'; "
+    "form-action 'none'; frame-ancestors 'self'; img-src 'self' data: https: http:; "
+    "style-src 'self' 'unsafe-inline'; font-src 'self' data: https:; media-src 'self'; "
+    "sandbox allow-popups allow-popups-to-escape-sandbox"
+)
+
+
+def _html(document: str) -> HTMLResponse:
+    return HTMLResponse(content=document, headers={"Content-Security-Policy": _MESSAGE_CSP})
+
+
 async def _owned_message(db: AsyncSession, user_id: uuid.UUID, message_id: uuid.UUID) -> Message:
     msg = await db.scalar(
         select(Message).where(Message.id == message_id, Message.user_id == user_id)
@@ -123,7 +140,7 @@ async def message_html(
 
     stale = html_part is not None and html_part.sanitizer_v < SANITIZER_VERSION
     if html_part is not None and not allow_remote and not stale:
-        return HTMLResponse(content=_wrap(html_part.content))
+        return _html(_wrap(html_part.content))
 
     # Re-sanitize the original HTML (image-approved sender, or a stale stored copy).
     # If the raw blob is unavailable, fall back to whatever is already stored.
@@ -142,13 +159,13 @@ async def message_html(
                     html_part.content = result.html  # persist the upgraded sanitization
                     html_part.sanitizer_v = result.sanitizer_v
                     await db.commit()
-                return HTMLResponse(content=_wrap(result.html))
+                return _html(_wrap(result.html))
 
     if html_part is not None:
-        return HTMLResponse(content=_wrap(html_part.content))
+        return _html(_wrap(html_part.content))
     text_part = next((p for p in parts if p.kind == "text"), None)
     body = html_lib.escape(text_part.content) if text_part else ""
-    return HTMLResponse(content=_wrap(f'<div class="wm-plaintext">{body}</div>'))
+    return _html(_wrap(f'<div class="wm-plaintext">{body}</div>'))
 
 
 @router.post("/{message_id}/allow-remote-images", dependencies=[Depends(require_csrf)])