Fix invoice-image code-review findings: aspect ratio, whitelist config, stale refs

- MigraDocExtensions: fix a bug where capping an oversized explicit
  width+height image re-derived the other dimension from the image's
  native pixel ratio instead of scaling both sides uniformly, silently
  distorting a deliberately non-native-ratio (e.g. stretched) image.
- InvoiceService: derive the mfr-portal image whitelist prefix from the
  existing Fds:MFR_host config setting (via MFRClientConfig.RestRoot)
  instead of a second hardcoded literal, so it can't drift from the
  actual configured host; parse <img> sources with HtmlAgilityPack
  (already used by the PDF's own HTML converter) instead of a regex, so
  a decoy attribute like data-src can't be picked up in place of the
  real src; dedup fetch URLs with a HashSet instead of O(n^2) List
  scanning; reuse FuchsPdf.MigraDocFilenameFromByteArray instead of
  re-spelling the "base64:" prefix.
- Fix a stale log message and comment left over from the
  GenerateInvoicePdf -> GenerateInvoicePdfAsync rename, and update
  INVOICE_LIFECYCLE.md's references to the old sync method name.
- Add tests for the config-derived whitelist, a decoy-attribute src,
  and the 20-image-per-invoice fetch cap boundary.

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

Fuchs/Docs/INVOICE_LIFECYCLE.md

@@ -116,7 +116,7 @@ vat }` plus set header/member tagging) that is posted to the server.
 | User action | JS entry point | Endpoint | Controller handler | Effect |
 |---|---|---|---|---|
 | First save without preview | `$inv.ssave` | `req/save` | `Do_Process_Requests` case `save` | `RegisterInvoiceAsync(change: id present)` — creates or updates the draft row, **no PDF rendered** |
-| Preview a **new** invoice | `$inv.sprev(false)` | `req/sprep` | case `sprep` | Registers draft, then `GenerateInvoicePdf` → renders preview images |
+| Preview a **new** invoice | `$inv.sprev(false)` | `req/sprep` | case `sprep` | Registers draft, then `GenerateInvoicePdfAsync` → renders preview images |
 | Preview an **existing** draft | `$inv.sprev(true)` (aka `$inv.sedit`) | `req/sedit` | case `sedit` | Same as `sprep` but requires `id`, updates existing draft |
 | Cancel out of preview | dialog `cancel` handler | `req/sdel` | case `sdel` | `fds__remInvoice` — deletes the (unfinalised) draft row |
 
@@ -134,7 +134,8 @@ JSON into SQL parameters and calls, in one batch:
 - **Existing draft**: `fds__setInvoice` (same parameter set, updates in place)
   → `fds__createInvoice_Details` again.
 
-`GenerateInvoicePdf` (still in `InvoiceService`) builds `FuchsPdf.FdsTextBlocks`
+`GenerateInvoicePdfAsync` (still in `InvoiceService`) builds `FuchsPdf.FdsTextBlocks`,
+resolves any whitelisted mfr-portal images from the line-item text (ADR 0018),
 and calls `IPdfService.WriteLetterAsync` + `ApplyInvoice`, which internally
 uses `InvoiceSetPricing.Build(...)` to turn the posted item contract into
 ordered, priced/unpriced lines. While the invoice is **not yet final**
@@ -160,7 +161,7 @@ sequenceDiagram
 	C->>S: RegisterInvoiceAsync(invoice, change, invId)
 	S->>DB: fds__createInvoice / fds__setInvoice + fds__createInvoice_Details
 	DB-->>S: registered invoice row (Id, ...)
-	S->>S: GenerateInvoicePdf (draft=true -> watermark, no GiroCode)
+	S->>S: GenerateInvoicePdfAsync (draft=true -> watermark, no GiroCode)
 	S-->>C: MigraDoc Document
 	C-->>JS: {id, img[] (base64 pages), total}
 	JS->>U: show PDF preview modal (confirm / cancel)