fix(dkv): extract invoice number/date from PDF, rename export files to RG-DKV format

- Parser now extracts Rechnungsnummer (DD/DDDDDDDDD/DDD) and Rechnungsdatum
  from PDF text, so filename doesn't rely on email subject
- Export filename changed from DKV_YYYY-MM_... to RG-DKV-{nr}-{YYMMDD}.xlsx
  e.g. RG-DKV-26-650869002-002-260331.xlsx
- Subject fallback now also matches slash-separated invoice numbers (26/NNN/NNN)
- writeAndPrune simplified to accept baseName instead of separate fields
- Validation regex and prune prefix updated to match new RG-DKV- pattern

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/dkv/dkv-parser.service.ts

@@ -31,7 +31,11 @@ export class DkvParserService {
    * @returns Array of vehicle blocks with transactions
    * @throws Error with generic message if no vehicle blocks are found
    */
-  async parsePdf(buffer: Buffer): Promise<DkvVehicleBlock[]> {
+  async parsePdf(buffer: Buffer): Promise<{
+    vehicles: DkvVehicleBlock[];
+    rechnungsnummer: string | null;
+    rechnungsdatum: string | null;
+  }> {
     let text: string;
     try {
       text = await this.extractText(buffer);
@@ -52,7 +56,30 @@ export class DkvParserService {
       `DKV PDF parsed: ${vehicles.length} vehicle(s), ${totalTx} transaction(s)`,
     );
 
-    return vehicles;
+    return {
+      vehicles,
+      rechnungsnummer: this.extractRechnungsnummer(text),
+      rechnungsdatum: this.extractRechnungsdatum(text),
+    };
+  }
+
+  /** Extract first DKV invoice number (DD/DDDDDDDDD/DDD) from PDF text. */
+  private extractRechnungsnummer(text: string): string | null {
+    const m = text.match(/\b(\d{2}\/\d{9}\/\d{3})\b/);
+    return m ? m[1] : null;
+  }
+
+  /** Extract invoice date ("DD.MM.YYYY") appearing near "Rechnungsdatum" in PDF text. */
+  private extractRechnungsdatum(text: string): string | null {
+    // "Rechnungsdatum" is followed (within ~200 chars) by a date string
+    const m = text.match(/Rechnungsdatum[:\s\t]*(\d{2}\.\d{2}\.\d{4})/);
+    if (m) return m[1];
+    // Fallback: second line after "Rechnungsdatum:" marker
+    const idx = text.indexOf('Rechnungsdatum');
+    if (idx === -1) return null;
+    const after = text.slice(idx, idx + 200);
+    const dm = after.match(/(\d{2}\.\d{2}\.\d{4})/);
+    return dm ? dm[1] : null;
   }
 
   // ─── Private: PDF text extraction ──────────────────────────────────────────