Parallelize invoice image fetch, add telemetry, generalize HtmlConverter

- InvoiceService.ResolveInlineImagesAsync now fetches an invoice's
  distinct whitelisted images concurrently (SemaphoreSlim-capped at 4)
  instead of one at a time. It ran inline on every invoice render,
  including repeated draft-preview renders, so N sequential mfr-portal
  round-trips could otherwise add N x latency to a user-facing request.
- Add fuchs.invoice.inlineimages.fetches (counter, tagged by outcome:
  success/http_error/bad_content_type/oversized/exception) and
  fuchs.invoice.inlineimages.resolve.duration (histogram) to
  FuchsTelemetry, matching the project's observability convention for
  outbound-HTTP operations.
- MigraDocExtensions.HtmlConverter's pre-resolved-image parameter is
  renamed inlineImages -> resolvedImages with generic docs: the
  converter fetches nothing and knows nothing about invoices — which
  images to resolve, and any whitelist/network policy, is entirely the
  caller's concern (InvoiceService, ADR 0018 today; any future caller
  tomorrow).
- Add tests asserting fetches actually overlap (not just faster) and
  that the new counter/histogram are recorded.
- Update ADR 0018 to describe the concurrent fetch, telemetry, and the
  generalized converter parameter.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Fuchs/code/MigraDocExtensions.cs

@@ -26,13 +26,14 @@ namespace MigraDoc.Extensions
 
         public static Section   AddHtml(this Section   section, string html)
             => section.Add(html, new MigraDoc.Extensions.Html.HtmlConverter());
-        /// <param name="inlineImages">Maps an &lt;img&gt; element's original <c>src</c> to a MigraDoc
-        /// image source ("base64:...") for images already fetched/vetted by the caller (see
-        /// InvoiceService.ResolveInlineImagesAsync / ADR 0018). Src values not present here are
-        /// dropped, matching the converter's long-standing behaviour for unsupported elements.</param>
+        /// <param name="resolvedImages">Maps an &lt;img&gt; element's original <c>src</c> to a MigraDoc
+        /// image source ("base64:...") for images the caller has already fetched/vetted — this converter
+        /// never fetches anything itself. A src not present here is dropped, matching the converter's
+        /// long-standing behaviour for unsupported elements. (One caller: InvoiceService resolves
+        /// whitelisted invoice-text image URLs this way — see ADR 0018.)</param>
         public static Cell      AddHtml(this Cell      cell,    string html,
-            IReadOnlyDictionary<string, string>? inlineImages = null)
-            => cell.Add(html,    new MigraDoc.Extensions.Html.HtmlConverter(inlineImages));
+            IReadOnlyDictionary<string, string>? resolvedImages = null)
+            => cell.Add(html,    new MigraDoc.Extensions.Html.HtmlConverter(resolvedImages));
         public static TextFrame AddHtml(this TextFrame frame,   string html)
             => frame.Add(html,   new MigraDoc.Extensions.Html.HtmlConverter());
 
@@ -62,15 +63,20 @@ namespace MigraDoc.Extensions
 
 namespace MigraDoc.Extensions.Html
 {
-    /// <summary>Converts a subset of HTML to MigraDoc document objects.</summary>
+    /// <summary>
+    /// Converts a subset of HTML to MigraDoc document objects. Fetches nothing itself: an
+    /// &lt;img&gt; only renders when its <c>src</c> is a key in the <c>resolvedImages</c> map the
+    /// caller supplies — any network/whitelist policy for resolving those images lives entirely
+    /// with the caller (e.g. <c>InvoiceService.ResolveInlineImagesAsync</c>, ADR 0018).
+    /// </summary>
     public class HtmlConverter : MigraDoc.Extensions.IConverter
     {
         private readonly IDictionary<string, Func<HtmlNode, DocumentObject, DocumentObject>> _handlers;
-        private readonly IReadOnlyDictionary<string, string>? _inlineImages;
+        private readonly IReadOnlyDictionary<string, string>? _resolvedImages;
 
-        public HtmlConverter(IReadOnlyDictionary<string, string>? inlineImages = null)
+        public HtmlConverter(IReadOnlyDictionary<string, string>? resolvedImages = null)
         {
-            _inlineImages = inlineImages;
+            _resolvedImages = resolvedImages;
             _handlers = new Dictionary<string, Func<HtmlNode, DocumentObject, DocumentObject>>();
             AddDefaultHandlers();
         }
@@ -120,7 +126,7 @@ namespace MigraDoc.Extensions.Html
             _handlers["img"] = (n, p) =>
             {
                 string src = n.GetAttributeValue("src", "");
-                if (src.Length == 0 || _inlineImages == null || !_inlineImages.TryGetValue(src, out var source))
+                if (src.Length == 0 || _resolvedImages == null || !_resolvedImages.TryGetValue(src, out var source))
                     return p;
                 var para = GetPara(p);
                 if (para == null) return p;