feat(invoices): remove client-facing PDF generation

Phase 1 / commit 11 of 14 — invoices are client-facing documents, and
per the new "no CRM-generated client-facing PDFs" rule (see the design
spec), the in-app pdfme rendering is removed entirely.

Future invoice rendering will use the deferred AcroForm-fill admin-
template feature: admin uploads a PDF template with named form fields,
CRM fills them with invoice data via pdf-lib. Same pattern as the
in-app EOI pathway. Tracked in BACKLOG.md.

Deleted:
  - src/lib/services/invoices.ts:generateInvoicePdf (60 LOC)
  - src/lib/pdf/templates/invoice-template.ts (entire pdfme template)
  - src/app/api/v1/invoices/[id]/generate-pdf/route.ts
  - src/components/invoices/invoice-pdf-preview.tsx (regenerate UI)
  - "PDF Preview" tab on invoice detail page
  - 5 now-unused imports in invoices.ts (files, ports, buildStoragePath,
    getStorageBackend, env)

sendInvoice() retained: still queues the send-invoice email job, still
flips status to "sent", still emits the socket event. The PDF-attach
step is gone — downstream consumers either render externally or wait
for the AcroForm-fill feature. The `pdfFileId` column on invoices stays
so existing rows don't break, just never gets written by this code path.

1319/1319 vitest green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/invoices/invoice-detail.tsx

@@ -23,7 +23,6 @@ import {
   SelectValue,
 } from '@/components/ui/select';
 import { Tabs, TabsContent, TabsList, TabsTrigger } from '@/components/ui/tabs';
-import { InvoicePdfPreview } from './invoice-pdf-preview';
 import { apiFetch } from '@/lib/api/client';
 import { toastError } from '@/lib/api/toast-error';
 import { useMobileChrome } from '@/components/layout/mobile/mobile-layout-provider';
@@ -218,7 +217,6 @@ export function InvoiceDetail({ invoiceId }: InvoiceDetailProps) {
         <TabsList>
           <TabsTrigger value="overview">Overview</TabsTrigger>
           <TabsTrigger value="expenses">Linked Expenses</TabsTrigger>
-          <TabsTrigger value="pdf">PDF Preview</TabsTrigger>
           <TabsTrigger value="payment">Payment</TabsTrigger>
         </TabsList>
 
@@ -360,11 +358,6 @@ export function InvoiceDetail({ invoiceId }: InvoiceDetailProps) {
           )}
         </TabsContent>
 
-        {/* PDF Preview */}
-        <TabsContent value="pdf" className="pt-4">
-          <InvoicePdfPreview invoiceId={invoiceId} pdfFileId={invoice.pdfFileId} />
-        </TabsContent>
-
         {/* Payment */}
         <TabsContent value="payment" className="pt-4">
           {invoice.status === 'paid' ? (