[Server] Inform client of login/registration rate limit timeout (#7468)

* [Security] Add per-address rate limiting for auth endpoints

Introduce a thread-safe RateLimiter that tracks attempts per key
(IP address) within a sliding time window, and wire it into the
authentication endpoints:

- Login: failed login attempts from an address are counted; once the
  configured maximum is exceeded within the window, further logins
  from that address are rejected with RespTooManyRequests. A
  successful login clears the failed attempts for that address.
- Registration: implement the previously stubbed
  tooManyRegistrationAttempts, limiting how many accounts can be
  created per address per window.
- Forgot-password: throttle both the email-request and the
  email-challenge paths per address.

New [security] settings with defaults:
  max_login_attempts_per_ip=5 / login_attempt_window_seconds=900
  max_registrations_per_ip=2 / registration_window_seconds=3600
  max_forgot_password_requests_per_ip=3 / forgot_password_window_seconds=3600

Adds unit tests for the RateLimiter (window limit, over-limit
blocking, clearing, per-key independence).

Took 3 minutes

* [Server] Fix rate limiter: disable semantics, draining window, bounded keys, pre-work login gate

- recordAttempt()/isBlocked() return false when maxAttempts <= 0 or windowSeconds <= 0, so the documented 'set to 0 to disable' actually disables instead of blocking every address permanently
- Keys at their limit are rejected without recording, so repeated attempts at the attacker's pace cannot keep sliding the window and holding the lockout open; a block now clears once the recorded attempts age out of windowSeconds
- Attempts map is bounded: an opportunistic prune (every 60s) erases keys whose attempts have fully aged out
- Login is throttled before the work: isLoginRateLimited() (a new virtual on Server, backed by isBlocked()) gates loginUser(), so a locked-out address no longer burns a database round trip and password verification per attempt
- recordAttemptAt()/isBlockedAt() time seams make the sliding-window behaviour deterministically testable; tests rewritten for the real semantics (no reliance on the maxAttempts=0 bug, no sleeps)

* [Server] Inform client of login/registration rate limit timeout

- Add retryAfterSeconds/retryAfterSecondsAt to RateLimiter to compute
  seconds until oldest attempt exits the sliding window
- Expose loginRateLimitRemainingSeconds via Server/Servatrice
- Include denied_end_time (absolute epoch) in RespTooManyRequests for
  login pre-auth and when transitioning to lockout after failed attempt
- Include denied_end_time in registration rate limit response
- Client: show retry time for login RespTooManyRequests
- Client: simplify registration RespTooManyRequests message to show
  retry time when provided
- Add test for retryAfterSecondsAt

* [Client] Restore registration guidance and drop duplicated rate limit reason

The registration RespTooManyRequests case now only shows the retry-after
copy when the server actually sent a denied_end_time; the per-email account
cap path (no end time) falls back to the previous guidance about already
having an account and contacting the server operator.

Drop the canned denied_reason_str from the new login rate limit branches so
the client no longer prints the same sentence twice.

* [Server] Exempt trusted_sources from auth rate limits

Addresses in security/trusted_sources bypass per-address limits, matching
the existing max_users_per_address bypass in initSession. Without this,
Webatrice behind local nginx (all clients as 127.0.0.1) would lock out
every web user after 5 mistyped passwords, and cap the whole server at
2 registrations / 3 forgot-password requests per window.

Adds Servatrice::isRateLimitExempt helper; recordFailedLogin,
isLoginRateLimited, recordRegistrationAttempt and
recordForgotPasswordAttempt all check it.

# Conflicts:
#	servatrice/src/servatrice.cpp

* [Server] Key login rate limit by (address, username)

A successful login cleared the whole per-IP failure history, so 4 bad
guesses plus 1 self-login (or guest UnknownUser login) from the same
address bypassed the limiter. Failures are now bucketed per
(address, lowercase username) and a success clears only its own pair.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

servatrice/src/servatrice.h

@@ -21,6 +21,7 @@
 #define SERVATRICE_H
 
 #include "metrics_registry.h"
+#include "ratelimiter.h"
 #include "smtpclient.h"
 
 #include <QByteArray>
@@ -204,6 +205,8 @@ private:
     std::shared_ptr<const Response_ReportStats> reportStatsCache;
     static constexpr int reportStatsCacheTtlSeconds = 60;
 
+    RateLimiter rateLimiter;
+
     mutable QMutex serverListMutex;
     QList<ServerProperties> serverList;
     void updateServerList();
@@ -362,6 +365,25 @@ public:
      */
     void watchWorkerThread(QThread *thread);
 
+    RateLimiter *getRateLimiter()
+    {
+        return &rateLimiter;
+    }
+    bool recordFailedLogin(const QString &ipAddress, const QString &userName) override;
+    bool isLoginRateLimited(const QString &ipAddress, const QString &userName) override;
+    int loginRateLimitRemainingSeconds(const QString &ipAddress, const QString &userName) override;
+    void clearFailedLogins(const QString &ipAddress, const QString &userName) override;
+    bool recordRegistrationAttempt(const QString &ipAddress);
+    bool recordForgotPasswordAttempt(const QString &ipAddress);
+    /// True when the address is in security/trusted_sources and therefore exempt from per-address limits.
+    bool isRateLimitExempt(const QString &ipAddress) const;
+    int getMaxLoginAttemptsPerIp() const;
+    int getLoginAttemptWindowSeconds() const;
+    int getMaxRegistrationsPerIp() const;
+    int getRegistrationWindowSeconds() const;
+    int getMaxForgotPasswordRequestsPerIp() const;
+    int getForgotPasswordWindowSeconds() const;
+
     bool islConnectionExists(int _serverId) const;
     void addIslInterface(int _serverId, IslInterface *interface);
     void removeIslInterface(int _serverId);