feat(mcp): currency param on create_article and update_article (#1184)

Fixes #1168. articles.currency exists in the DB and REST API, but the
staged-operation schemas and the two MCP tools had no currency param,
so agent-created articles were always SEK and an agent asked to create
an EUR-priced article could not.

- CreateArticleParamsSchema/UpdateArticleParamsSchema accept an
  optional ISO 4217 code (normalized to upper case; empty/null =
  unset). The currencies-table FK stays the allow-list: a 23503 on the
  currency FK maps to a clear 400 instead of a raw 500.
- commitCreateArticle inserts currency ?? 'SEK'; the sparse update
  executor passes it through only when staged.
- gnubok_create_article / gnubok_update_article expose the param.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/pending-operations/commit.ts

@@ -488,6 +488,7 @@ async function commitCreateArticle(
       type: validated.type,
       unit: validated.unit ?? 'st',
       price_excl_vat: validated.price_excl_vat,
+      currency: validated.currency ?? 'SEK',
       vat_rate: validated.vat_rate,
       revenue_account: validated.revenue_account ?? null,
       cost_price: validated.cost_price ?? null,
@@ -499,7 +500,13 @@ async function commitCreateArticle(
     .select()
     .single()
 
-  if (error) return { error: error.message, status: 500 }
+  if (error) {
+    // FK to public.currencies: the reference table is the allow-list.
+    if (error.code === '23503' && error.message.includes('currency')) {
+      return { error: `Currency ${validated.currency} is not supported`, status: 400 }
+    }
+    return { error: error.message, status: 500 }
+  }
 
   if (!data.article_number) {
     try {
@@ -552,6 +559,10 @@ async function commitUpdateArticle(
 
   if (error) {
     if (error.code === 'PGRST116') return { error: 'Article not found', status: 404 }
+    // FK to public.currencies: the reference table is the allow-list.
+    if (error.code === '23503' && error.message.includes('currency')) {
+      return { error: `Currency ${validated.currency} is not supported`, status: 400 }
+    }
     return { error: error.message, status: 500 }
   }