feat(accounting): re-categorize incoming invoices, note field, pending re-bill pool

Address three incoming-invoice issues:

1. Re-categorization: a categorized invoice can now be changed again (e.g.
   passthrough → company expense). New "Re-categorize" button pre-fills the
   triage modal from the existing disposition/customer/markup/note.
   categorizeInbound is re-runnable — it unwinds any prior re-bill line
   (removes the invoice line + recomputes totals) before applying the new
   disposition, and refuses (INVOICE_LOCKED) when the re-bill is on an
   already-issued invoice.

2. Note field: new `note` column (migration 132 — 126 is already on beta)
   captured in triage and shown in the read-only view.

3. Re-bill like hours: rebill/passthrough now persist customer_account_id.
   Per-event customers accumulate as PENDING items, surfaced in a new
   "Pending re-bills" card and bundled into one invoice via "Bill these"
   (mirrors unbilled-hours billing). Monthly/manual customers keep
   auto-consolidating onto their running draft. Passthrough (durchlaufend)
   can now also attach to a customer with optional markup.

Adds backend unit tests for buildInboundLineItem + isInvoiceMutable and
en/de translations (other locales fall back to English defaults).

frontend/src/i18n/locales/de.json

@@ -3520,6 +3520,7 @@
       "saveCategorize": "Speichern",
       "saveCategorizePay": "Speichern & als bezahlt markieren",
       "categorize": "Kategorisieren",
+      "recategorize": "Neu kategorisieren",
       "view": "Ansehen",
       "empty": "Noch keine Dokumente — oben eines erfassen.",
       "untitled": "Unbenanntes Dokument",
@@ -3551,7 +3552,10 @@
         "eventId": "Event-ID (optional)",
         "markup": "Zuschlag",
         "reference": "Zahlungsreferenz",
-        "referenceHint": "QR-/ESR-Referenz oder Mitteilung"
+        "referenceHint": "QR-/ESR-Referenz oder Mitteilung",
+        "note": "Notiz",
+        "noteHint": "Interne Notiz zu dieser Rechnung (optional)",
+        "passthroughCustomerHint": "Optional — einen Kunden zuordnen, um diese durchlaufende Position weiterzuverrechnen; leer lassen, um sie nur auf das Event zu buchen."
       }
     },
     "expenseStatus": {
@@ -3584,7 +3588,15 @@
       "paid": "Bezahlt",
       "paidToast": "Als bezahlt markiert.",
       "categorizedToast": "Kategorisiert.",
-      "categorizedPaidToast": "Kategorisiert und als bezahlt markiert."
+      "categorizedPaidToast": "Kategorisiert und als bezahlt markiert.",
+      "pendingRebill": "Weiterverrechnung offen",
+      "pendingTitle": "Offene Weiterverrechnungen",
+      "pendingBody": "Kategorisierte Rechnungen, die auf die Weiterverrechnung warten. Posten eines Kunden zu einer Rechnung bündeln.",
+      "pendingCount": "{{count}} Posten",
+      "pendingCount_other": "{{count}} Posten",
+      "billPending": "Verrechnen",
+      "bundledToast": "{{count}} Weiterverrechnung zu einer Rechnung gebündelt.",
+      "bundledToast_other": "{{count}} Weiterverrechnungen zu einer Rechnung gebündelt."
     },
     "expense": {
       "kind": "Art",

frontend/src/i18n/locales/en.json

@@ -3520,6 +3520,7 @@
       "saveCategorize": "Save",
       "saveCategorizePay": "Save & mark paid",
       "categorize": "Categorize",
+      "recategorize": "Re-categorize",
       "view": "View",
       "empty": "No documents yet — capture one above.",
       "untitled": "Untitled document",
@@ -3551,7 +3552,10 @@
         "eventId": "Event ID (optional)",
         "markup": "Markup",
         "reference": "Payment reference",
-        "referenceHint": "QR / ESR reference or message"
+        "referenceHint": "QR / ESR reference or message",
+        "note": "Note",
+        "noteHint": "Internal note for this invoice (optional)",
+        "passthroughCustomerHint": "Optional — attach a client to re-bill this passthrough; leave empty to only book it to the event."
       }
     },
     "expenseStatus": {
@@ -3584,7 +3588,15 @@
       "paid": "Paid",
       "paidToast": "Marked as paid.",
       "categorizedToast": "Categorized.",
-      "categorizedPaidToast": "Categorized and marked paid."
+      "categorizedPaidToast": "Categorized and marked paid.",
+      "pendingRebill": "Pending re-bill",
+      "pendingTitle": "Pending re-bills",
+      "pendingBody": "Categorized invoices waiting to be re-billed. Bundle a client’s items into one invoice.",
+      "pendingCount": "{{count}} item",
+      "pendingCount_other": "{{count}} items",
+      "billPending": "Bill these",
+      "bundledToast": "Bundled {{count}} re-bill into one invoice.",
+      "bundledToast_other": "Bundled {{count}} re-bills into one invoice."
     },
     "expense": {
       "kind": "Type",