feat(filename): support compound last names like de Gruyter

Replace the four fixed regexes with a split-based algorithm:
- first segment = date → last segment = firstName, rest = lastName parts
- last segment = date → second-to-last = firstName, rest = lastName parts

18881025_de_Gruyter_Walter.pdf now correctly yields "Walter de Gruyter".
Simple two-segment names behave identically to before.

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

frontend/src/lib/utils/filename.spec.ts

@@ -2,8 +2,8 @@ import { describe, it, expect } from 'vitest';
 import { parseFilename, stripExtension } from './filename';
 
 describe('parseFilename', () => {
-	describe('YYYY-MM-DD_Lastname_Firstname pattern', () => {
-		it('extracts date and name', () => {
+	describe('date-first patterns', () => {
+		it('YYYY-MM-DD_Lastname_Firstname', () => {
 			expect(parseFilename('1965-03-12_Mueller_Hans.pdf')).toEqual({
 				dateIso: '1965-03-12',
 				personName: 'Hans Mueller',
@@ -11,40 +11,52 @@ describe('parseFilename', () => {
 			});
 		});
 
+		it('YYYYMMDD_Lastname_Firstname', () => {
+			expect(parseFilename('19650312_Mueller_Hans.pdf')).toEqual({
+				dateIso: '1965-03-12',
+				personName: 'Hans Mueller',
+				suggestedTitle: 'Hans Mueller (12.03.1965)'
+			});
+		});
+
+		it('YYYYMMDD_compound_lastname_Firstname', () => {
+			expect(parseFilename('18881025_de_Gruyter_Walter.pdf')).toEqual({
+				dateIso: '1888-10-25',
+				personName: 'Walter de Gruyter',
+				suggestedTitle: 'Walter de Gruyter (25.10.1888)'
+			});
+		});
+
 		it('handles umlauts in names', () => {
 			const result = parseFilename('2024-01-15_Müller_Jürgen.pdf');
 			expect(result.personName).toBe('Jürgen Müller');
 		});
 	});
 
-	describe('YYYYMMDD_Lastname_Firstname pattern', () => {
-		it('extracts date and name', () => {
-			expect(parseFilename('19650312_Mueller_Hans.pdf')).toEqual({
-				dateIso: '1965-03-12',
-				personName: 'Hans Mueller',
-				suggestedTitle: 'Hans Mueller (12.03.1965)'
-			});
-		});
-	});
-
-	describe('Lastname_Firstname_YYYY-MM-DD pattern', () => {
-		it('extracts date and name', () => {
+	describe('date-last patterns', () => {
+		it('Lastname_Firstname_YYYY-MM-DD', () => {
 			expect(parseFilename('Mueller_Hans_1965-03-12.pdf')).toEqual({
 				dateIso: '1965-03-12',
 				personName: 'Hans Mueller',
 				suggestedTitle: 'Hans Mueller (12.03.1965)'
 			});
 		});
-	});
 
-	describe('Lastname_Firstname_YYYYMMDD pattern', () => {
-		it('extracts date and name', () => {
+		it('Lastname_Firstname_YYYYMMDD', () => {
 			expect(parseFilename('Mueller_Hans_19650312.pdf')).toEqual({
 				dateIso: '1965-03-12',
 				personName: 'Hans Mueller',
 				suggestedTitle: 'Hans Mueller (12.03.1965)'
 			});
 		});
+
+		it('compound_lastname_Firstname_YYYYMMDD', () => {
+			expect(parseFilename('de_Gruyter_Walter_18881025.pdf')).toEqual({
+				dateIso: '1888-10-25',
+				personName: 'Walter de Gruyter',
+				suggestedTitle: 'Walter de Gruyter (25.10.1888)'
+			});
+		});
 	});
 
 	describe('non-matching filenames', () => {
@@ -52,7 +64,7 @@ describe('parseFilename', () => {
 			expect(parseFilename('1965-03-12.pdf')).toEqual({});
 		});
 
-		it('returns empty for name-only filename', () => {
+		it('returns empty for two segments with no date', () => {
 			expect(parseFilename('Mueller_Hans.pdf')).toEqual({});
 		});
 
@@ -60,13 +72,17 @@ describe('parseFilename', () => {
 			expect(parseFilename('scan_001.pdf')).toEqual({});
 		});
 
-		it('returns empty for three name segments without date', () => {
+		it('returns empty for three name segments with no date', () => {
 			expect(parseFilename('Mueller_Hans_Juergen.pdf')).toEqual({});
 		});
 
 		it('returns empty for filename without extension', () => {
 			expect(parseFilename('1965-03-12_Mueller_Hans')).toEqual({});
 		});
+
+		it('rejects implausible date (month 13)', () => {
+			expect(parseFilename('19651345_Mueller_Hans.pdf')).toEqual({});
+		});
 	});
 });