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/Observability/FuchsTelemetry.cs

@@ -54,6 +54,9 @@ public static class FuchsTelemetry
     public static readonly Counter<long> SystemProbes =
         Meter.CreateCounter<long>("fuchs.systemstatus.probes", "{probe}",
             "Admin system-status connectivity probes executed, tagged by component and outcome.");
+    public static readonly Counter<long> InvoiceInlineImageFetches =
+        Meter.CreateCounter<long>("fuchs.invoice.inlineimages.fetches", "{fetch}",
+            "Invoice-text image fetches for inline PDF rendering (ADR 0018), tagged by outcome.");
 
     // ── Performance histograms (durations in milliseconds) ───────────────────
     public static readonly Histogram<double> PdfRenderDuration =
@@ -62,6 +65,9 @@ public static class FuchsTelemetry
         Meter.CreateHistogram<double>("fuchs.report.render.duration", "ms", "Report render duration.");
     public static readonly Histogram<double> EmailSendDuration =
         Meter.CreateHistogram<double>("fuchs.email.send.duration", "ms", "Email send round-trip duration.");
+    public static readonly Histogram<double> InvoiceInlineImagesResolveDuration =
+        Meter.CreateHistogram<double>("fuchs.invoice.inlineimages.resolve.duration", "ms",
+            "Total wall-clock time to fetch an invoice's inline images (all images in parallel).");
 
     /// <summary>Starts a span on the application's <see cref="ActivitySource"/>.</summary>
     public static Activity? StartActivity(string name, ActivityKind kind = ActivityKind.Internal) =>