fix(crowdsec): send a payload LAPI accepts (500 on alerts, 401 on auth) (#2553)

## Problem

Contributing a ban to CrowdSec failed against a real LAPI — `POST /v1/alerts` answered **500**, and depending on the shard's locale, auth answered **401**. Three independent defects, each sufficient on its own.

## Fixes

**`scenario_hash` / `scenario_version` were never serialized.** LAPI dereferences both unconditionally when persisting an alert, so omitting them is a nil deref and a 500 rather than a validation error. Both are now emitted with the values a watcher without a hub scenario is expected to send (`""` and `"1.0"`).

**`start_at`/`stop_at` were formatted without an `IFormatProvider`.** `:` is the time separator *specifier* in a custom .NET format string, not a literal — a shard running under a culture like `fi-FI` emitted `T15.04.05.123Z`, which Go's `time.RFC3339` rejects, producing another 500. Non-Gregorian cultures (`th-TH`, `ar-SA`) would also shift the year. Formatting is now pinned to `InvariantCulture` in `FormatTimestamp`, which additionally converts non-UTC input — the trailing `Z` is a literal and was previously an unchecked claim.

**The `User-Agent` was a plain product string.** LAPI's default watcher profile matches the `crowdsec/` prefix and answers 401 without it, so the header is a protocol constraint, not cosmetic. It is now an `internal const` carrying that reason.

Also fixed, same root cause as the timestamp bug: the login-expiry parse used a bare `DateTime.TryParse` on LAPI's RFC3339 `expire`. Under a mismatched culture that silently fails and falls back to a fabricated `UtcNow + 1h`, pushing re-auth past the real expiry and costing a 401-relogin round trip on every send.

`capacity` now defaults to `1` instead of `0`, matching the one-decision-per-alert shape actually being sent.

## Note on scope

The two 500 causes are independent. On an `en-US` shard only the missing scenario fields were biting; the date bug was latent and would have surfaced as an unexplained regression the first time someone ran a shard under a European locale.

## Verification

The emitted payload is field-for-field identical to a hand-verified request that a live LAPI accepts:

```json
[
  {
    "scenario": "modernuo/rate-limit",
    "scenario_hash": "",
    "scenario_version": "1.0",
    "message": "ModernUO rate-limit ban for 192.0.2.123",
    "events_count": 1,
    "start_at": "2026-07-27T15:04:05.123Z",
    "stop_at": "2026-07-27T15:04:05.123Z",
    "capacity": 1,
    "leakspeed": "0s",
    "simulated": false,
    "events": [],
    "remediation": true,
    "source": { "scope": "Ip", "value": "192.0.2.123" },
    "decisions": [
      {
        "origin": "modernuo",
        "type": "ban",
        "scope": "Ip",
        "value": "192.0.2.123",
        "duration": "300s",
        "scenario": "modernuo/rate-limit"
      }
    ]
  }
]
```

Regression tests assert the required scenario fields on the **serialized JSON** rather than the DTO — the DTO is not what goes on the wire — and cover the timestamp as a `[Theory]` across `fi-FI`/`th-TH`/`ar-SA`.

`dotnet test --filter "FullyQualifiedName~CrowdSec"` → **21/21 passed**, build clean with 0 warnings.

Projects/UOContent/Misc/CrowdSec/CrowdSecAlertClient.cs

@@ -15,6 +15,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Net;
 using System.Net.Http;
 using System.Net.Http.Headers;
@@ -40,6 +41,12 @@ public sealed class CrowdSecAlertClient : ICrowdSecAlertClient
 {
     private static readonly JsonSerializerOptions _jsonOptions = new() { PropertyNameCaseInsensitive = true };
 
+    /// <summary>
+    /// The <c>crowdsec/</c> prefix is load-bearing: LAPI's default watcher profile matches on it and
+    /// answers 401 for anything else, so this cannot be a plain product string.
+    /// </summary>
+    internal const string UserAgent = "crowdsec/ModernUO-watcher-1.0";
+
     private readonly HttpClient _http;
     private readonly string _machineId;
     private readonly string _password;
@@ -51,7 +58,7 @@ public sealed class CrowdSecAlertClient : ICrowdSecAlertClient
     {
         var baseUri = new Uri(settings.LapiUrl, UriKind.Absolute); // fails loud on malformed url
         _http = new HttpClient { BaseAddress = baseUri, Timeout = TimeSpan.FromSeconds(30) };
-        _http.DefaultRequestHeaders.Add("User-Agent", "ModernUO-watcher/1.0");
+        _http.DefaultRequestHeaders.Add("User-Agent", UserAgent);
         _machineId = settings.MachineId;
         _password = settings.Password;
     }
@@ -71,7 +78,15 @@ public sealed class CrowdSecAlertClient : ICrowdSecAlertClient
         var login = await response.Content.ReadFromJsonAsync<CrowdSecLoginResponse>(_jsonOptions, token)
             .ConfigureAwait(false);
         _token = login?.Token ?? throw new InvalidOperationException("CrowdSec login returned no token.");
-        _tokenExpiresUtc = DateTime.TryParse(login.Expire, out var exp) ? exp.ToUniversalTime() : DateTime.UtcNow.AddHours(1);
+
+        // LAPI returns an RFC3339 expiry. Parse it invariantly for the same reason we format invariantly:
+        // the current culture must not decide whether a machine-readable timestamp is understood.
+        _tokenExpiresUtc = DateTime.TryParse(
+            login.Expire,
+            CultureInfo.InvariantCulture,
+            DateTimeStyles.AdjustToUniversal,
+            out var exp
+        ) ? exp : DateTime.UtcNow.AddHours(1);
     }
 
     private void Authorize(HttpRequestMessage message) =>