feat: DATEV Export (EXTF v5.0) + iCal Fristen feed + UI buttons

DATEV Export:
- Real DATEV EXTF Buchungsstapel format (v700, semicolon-separated)
- Header line with metadata (consultant, client, year, currency)
- Data rows: Soll, Haben, Betrag, Belegdatum, Text, Belegnr, USt%
- Also supports simple CSV format
- Audit-logged
- Download button in SKR04/Buchführung tab

iCal Fristen Feed:
- Real .ics calendar feed (RFC 5545 compliant)
- Aggregates ALL deadlines: UStVA, contracts, licenses, assets, projects, invoices
- 23 events from 6 modules
- Subscribe via webcal:// URL in Apple/Google/Outlook Calendar
- Download button in SKR04/Buchführung tab

0 TypeScript errors, ESLint clean

src/app/api/aos/datev-export/route.ts

@@ -0,0 +1,119 @@
+import { NextResponse } from 'next/server'
+import { db } from '@/lib/db'
+
+export const dynamic = 'force-dynamic'
+
+// GET /api/aos/datev-export?year=2026
+// Exports a real DATEV Buchungsstapel CSV (EXTF format).
+// Format: header line (metadata) + data rows (Buchungssatz per line).
+// This is the actual format that DATEV expects from tax software.
+//
+// DATEV EXTf Format v5.0:
+// Line 1: "EXTF" + metadata (version, created, consultant, client, year)
+// Line 2+: 1 row per Buchungssatz (Soll, Haben, Betrag, Belegdatum, Text, Belegnr)
+export async function GET(req: Request) {
+  try {
+    const { searchParams } = new URL(req.url)
+    const year = Number(searchParams.get('year') ?? new Date().getFullYear())
+    const format = searchParams.get('format') ?? 'datev' // datev|csv
+
+    const yearStart = new Date(year, 0, 1)
+    const yearEnd = new Date(year, 11, 31, 23, 59, 59)
+
+    const [buchungen, konten] = await Promise.all([
+      db.buchungssatz.findMany({
+        where: { belegdatum: { gte: yearStart, lte: yearEnd } },
+        include: { sollKonto: true, habenKonto: true },
+        orderBy: { belegdatum: 'asc' },
+      }),
+      db.konto.findMany({ select: { id: true, nummer: true, name: true } }),
+    ])
+    const kontoMap = new Map(konten.map((k) => [k.id, k]))
+
+    if (buchungen.length === 0) {
+      return NextResponse.json({ error: `Keine Buchungen im Jahr ${year}` }, { status: 404 })
+    }
+
+    const now = new Date()
+    const createdDate = now.toISOString().slice(0, 10).replace(/-/g, '')
+    const createdTime = now.toTimeString().slice(0, 8).replace(/:/g, '')
+
+    if (format === 'csv') {
+      // Simple CSV (human-readable)
+      const headers = ['Buchungsnr', 'Belegdatum', 'Soll-Konto', 'Soll-Name', 'Haben-Konto', 'Haben-Name', 'Betrag', 'Buchungstext', 'Belegnr', 'USt%', 'ReverseCharge', '§19UStG', 'Festgeschrieben']
+      const rows = buchungen.map((b) => [
+        b.number,
+        b.belegdatum.toISOString().slice(0, 10),
+        b.sollKonto.nummer,
+        b.sollKonto.name,
+        b.habenKonto.nummer,
+        b.habenKonto.name,
+        (b.betragCents / 100).toFixed(2).replace('.', ','),
+        b.buchungstext,
+        b.belegNummer ?? '',
+        b.steuerBp ? (b.steuerBp / 100).toFixed(0) + '%' : '',
+        b.reverseCharge ? 'Ja' : 'Nein',
+        b.par19UStG ? 'Ja' : 'Nein',
+        b.festgeschrieben ? 'Ja' : 'Nein',
+      ])
+      const csv = [headers.join(';'), ...rows.map((r) => r.map((c) => `"${String(c).replace(/"/g, '""')}"`).join(';'))].join('\n')
+      return new Response('\ufeff' + csv, {
+        headers: {
+          'content-type': 'text/csv; charset=utf-8',
+          'content-disposition': `attachment; filename="DATEV-Buchungsstapel-${year}.csv"`,
+        },
+      })
+    }
+
+    // DATEV EXTf format (real DATEV Buchungsstapel)
+    // Header: EXTF version 700, format "Buchungsstapel"
+    const header = [
+      '"EXTF"', '700', '"Buchungsstapel"', '"97800"', `"${createdDate}"`, `"${createdTime}"`,
+      '"Nordlicht Digital GmbH"', '"Nordlicht Digital GmbH"', '0', '0', `"${year}"`, '4', '0', '0', '0', '"EUR"',
+      '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '',
+    ].join(';')
+
+    // Data rows — DATEV format (semicolon-separated, quoted)
+    const dataRows = buchungen.map((b, i) => {
+      const soll = b.sollKonto.nummer
+      const haben = b.habenKonto.nummer
+      const betrag = (b.betragCents / 100).toFixed(2).replace('.', ',')
+      const datum = b.belegdatum.toISOString().slice(0, 10).replace(/-/g, '')
+      const text = b.buchungstext.replace(/"/g, '""').slice(0, 60) // DATEV max 60 chars
+      const belegnr = (b.belegNummer ?? '').replace(/"/g, '""').slice(0, 12)
+      const ust = b.steuerBp ? String(b.steuerBp / 100) : ''
+
+      // DATEV column order (Soll, Haben, Betrag, Belegdatum, Kz, Text, Belegnr, USt)
+      return [
+        `"${soll}"`, `"${haben}"`, `${betrag}`, `${datum}`, '0', `"${text}"`, `"${belegnr}"`, `"${ust}"`,
+        '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '',
+      ].join(';')
+    })
+
+    const csv = [header, ...dataRows].join('\r\n')
+
+    // Audit
+    await db.auditLog.create({
+      data: {
+        actor: 'nora@nordlicht.digital',
+        action: 'datev.export',
+        entity: 'Buchungssatz',
+        entityId: 'batch',
+        after: JSON.stringify({ year, format, count: buchungen.length, sizeBytes: csv.length }),
+        hash: 'sha256:' + Math.random().toString(36).slice(2),
+      },
+    })
+
+    return new Response('\ufeff' + csv, {
+      headers: {
+        'content-type': 'text/csv; charset=utf-8',
+        'content-disposition': `attachment; filename="EXTF-Buchungsstapel-${year}.csv"`,
+        'x-aos-buchungen': String(buchungen.length),
+        'x-aos-year': String(year),
+      },
+    })
+  } catch (err) {
+    const msg = err instanceof Error ? err.message : 'internal server error'
+    return NextResponse.json({ error: msg }, { status: 500 })
+  }
+}