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/identity/password_policy.py

@@ -0,0 +1,145 @@
+"""What a new account password must not be (sign-up and reset).
+
+Length is bounded by the request schemas (8–128). On top of that a password is
+refused when it is one of the passwords every guessing attack tries first, is
+made of the user's own address, is one character repeated, or has turned up in
+a known data breach. The breach check asks Have I Been Pwned's range API with
+the first five hex digits of the password's SHA-1 only (k-anonymity: neither the
+password nor its hash leaves the server) and is skipped, not failed, when that
+service cannot be reached — a sign-up never hangs on someone else's outage.
+Set ``PASSWORD_BREACH_CHECK=false`` to never ask it.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import logging
+
+import httpx
+
+from app.config import get_settings
+
+logger = logging.getLogger(__name__)
+
+MIN_LENGTH = 8
+MAX_LENGTH = 128
+
+_RANGE_API = "https://api.pwnedpasswords.com/range/"
+_TIMEOUT = 3.0
+
+# The front of every guessing list (and of every breach), lower-cased.
+_COMMON = frozenset(
+    {
+        "password",
+        "password1",
+        "password12",
+        "password123",
+        "password1234",
+        "passw0rd",
+        "p@ssw0rd",
+        "p@ssword",
+        "12345678",
+        "123456789",
+        "1234567890",
+        "0123456789",
+        "87654321",
+        "11111111",
+        "00000000",
+        "12341234",
+        "qwertyui",
+        "qwerty123",
+        "qwerty1234",
+        "qwertyuiop",
+        "1q2w3e4r",
+        "1q2w3e4r5t",
+        "1qaz2wsx",
+        "zaq12wsx",
+        "asdfghjk",
+        "asdfasdf",
+        "iloveyou",
+        "iloveyou1",
+        "sunshine",
+        "princess",
+        "football",
+        "baseball",
+        "welcome1",
+        "welcome123",
+        "letmein1",
+        "trustno1",
+        "superman",
+        "batman123",
+        "dragon123",
+        "monkey123",
+        "abc12345",
+        "abcd1234",
+        "aa123456",
+        "qazwsxedc",
+        "changeme",
+        "changeme1",
+        "administrator",
+        "admin123",
+        "adminadmin",
+        "computer",
+        "internet",
+        "whatever",
+        "starwars",
+        "michelle",
+        "jennifer",
+        "charlie1",
+        "zaq1zaq1",
+        "haslo123",
+        "polska123",
+        "kochamcie",
+        "zaq1@wsx",
+        "qwerty12",
+        "1234qwer",
+        "q1w2e3r4",
+    }
+)
+
+
+class PasswordRejected(Exception):
+    """The password may not be used; ``code`` and ``message`` say why."""
+
+    def __init__(self, code: str, message: str) -> None:
+        super().__init__(message)
+        self.code = code
+        self.message = message
+
+
+async def _breached(password: str) -> bool:
+    digest = hashlib.sha1(password.encode("utf-8")).hexdigest().upper()  # noqa: S324 - HIBP's format
+    prefix, suffix = digest[:5], digest[5:]
+    try:
+        async with httpx.AsyncClient(timeout=_TIMEOUT) as client:
+            resp = await client.get(_RANGE_API + prefix, headers={"Add-Padding": "true"})
+            resp.raise_for_status()
+    except httpx.HTTPError as exc:
+        logger.warning("breached-password check skipped: %s", exc)
+        return False
+    for line in resp.text.splitlines():
+        candidate, _, count = line.partition(":")
+        if candidate.strip() == suffix and count.strip() not in ("", "0"):  # padding rows say 0
+            return True
+    return False
+
+
+async def check_new_password(password: str, handle: str) -> None:
+    """Raise :class:`PasswordRejected` if ``password`` may not be used by ``handle``."""
+    lowered = password.lower()
+    if len(set(password)) <= 2:
+        raise PasswordRejected("weak_password", "this password is too easy to guess")
+    if lowered in _COMMON:
+        raise PasswordRejected("weak_password", "this is one of the most common passwords")
+    localpart = handle.split("@", 1)[0].lower()
+    if len(localpart) >= 3 and localpart in lowered:
+        # The name with a year or a few characters added ("jsmith2026") is the
+        # first thing anyone who knows the address will try.
+        rest = lowered.replace(localpart, "", 1)
+        if len(rest) < 4 or all(not c.isalpha() for c in rest):
+            raise PasswordRejected("weak_password", "the password must not be your address")
+    if get_settings().password_breach_check and await _breached(password):
+        raise PasswordRejected(
+            "breached_password",
+            "this password has appeared in a data breach — please choose another",
+        )