Escape and fold the iCal export, serve it as text/calendar, add subscription links

- iCal export: RFC 5545 text escaping for SUMMARY, DESCRIPTION and LOCATION,
  line folding at 75 octets (UTF-8 aware), CRLF line endings, and
  Content-Type text/calendar instead of the inferred text/html. A line break
  in a description used to end the property early; the new calendar
  frontend has a multi-line description field.
- formatDate no longer shifts the event's end date in place.
- GET /calendar/events/subscriptions: the iCal URL of every calendar, with
  its shared password where needed, for the calendar app's "Abonnieren"
  dialog. Editors only (requireAppAccess('calendar')), the shared password
  is not accepted there, never cached; calendars without a configured
  credential are left out.
- Tests for both; documented in docs/calendar-ical.md.

Backwards compatible with the current Angular calendar app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/calendar/icalgenerator.service.test.ts

@@ -0,0 +1,111 @@
+import {describe, expect, it} from 'vitest';
+
+import {convertToIcal, escapeText, foldLine} from '../../src/models/calendar/events/icalgenerator.service.js';
+import {Event} from '../../src/models/calendar/events/event.interface.js';
+
+/**
+ * The iCal export is read by every subscribed calendar app, so a malformed
+ * file is an outage nobody on our side sees. These pin RFC 5545's text
+ * escaping and line folding, which the generator did not do before: a line
+ * break in a description ended the DESCRIPTION property early, and the new
+ * calendar frontend's multi-line description field would have produced
+ * exactly that.
+ */
+
+const event = (overrides: Partial<Event> = {}): Event => ({
+	eventId: 1,
+	calendarId: 1,
+	uuid: 'uuid-1',
+	name: 'Konzert',
+	description: '',
+	startDateTime: new Date('2026-10-03T17:00:00Z'),
+	endDateTime: new Date('2026-10-03T19:00:00Z'),
+	createdDate: new Date('2026-01-01T12:00:00Z'),
+	location: '',
+	createdBy: 'Anna',
+	url: '',
+	wholeDay: false,
+	repeatFrequency: '',
+	...overrides
+});
+
+/** RFC 5545 unfolding: a CRLF followed by one space or tab joins the lines. */
+const unfold = (ical: string): string[] => ical.replace(/\r\n[ \t]/g, '').split('\r\n').filter(Boolean);
+
+describe('escapeText', () => {
+	it('escapes backslash, semicolon, comma and line breaks', () => {
+		expect(escapeText('a\\b;c,d')).toBe('a\\\\b\\;c\\,d');
+		expect(escapeText('Zeile 1\nZeile 2\r\nZeile 3\rZeile 4')).toBe('Zeile 1\\nZeile 2\\nZeile 3\\nZeile 4');
+	});
+
+	it('escapes the backslash first, so the others are not doubled', () => {
+		expect(escapeText(';')).toBe('\\;');
+	});
+});
+
+describe('foldLine', () => {
+	it('leaves a short line alone', () => {
+		expect(foldLine('SUMMARY:Konzert')).toBe('SUMMARY:Konzert');
+	});
+
+	it('keeps every physical line within 75 octets and unfolds back to the original', () => {
+		const line = 'DESCRIPTION:' + 'Probe im Gemeindehaus Süd – bitte Noten mitbringen. '.repeat(6);
+		const folded = foldLine(line);
+		for (const physical of folded.split('\r\n')) {
+			expect(Buffer.byteLength(physical, 'utf8')).toBeLessThanOrEqual(75);
+		}
+		expect(folded.replace(/\r\n /g, '')).toBe(line);
+	});
+
+	it('never splits a multi-byte character', () => {
+		const line = 'SUMMARY:' + 'ü'.repeat(80);
+		for (const physical of foldLine(line).split('\r\n')) {
+			expect(physical.replace(/^ /, '')).toMatch(/^(SUMMARY:)?ü*$/);
+			expect(Buffer.byteLength(physical, 'utf8')).toBeLessThanOrEqual(75);
+		}
+	});
+});
+
+describe('convertToIcal', () => {
+	it('terminates every line with CRLF', async () => {
+		const ical = await convertToIcal([event()]);
+		expect(ical.endsWith('END:VCALENDAR\r\n')).toBe(true);
+		expect(ical.replace(/\r\n/g, '')).not.toContain('\n');
+	});
+
+	it('keeps a multi-line description inside one DESCRIPTION property', async () => {
+		const ical = await convertToIcal([event({description: 'Einlass 18:30\nBeginn 19:00; Eintritt frei, Spenden willkommen'})]);
+		const lines = unfold(ical);
+		expect(lines).toContain('DESCRIPTION:Einlass 18:30\\nBeginn 19:00\\; Eintritt frei\\, Spenden willkommen');
+		// Nothing leaked out as a line of its own.
+		expect(lines.some((l) => l.startsWith('Beginn'))).toBe(false);
+	});
+
+	it('escapes the title and the location too', async () => {
+		const lines = unfold(await convertToIcal([event({name: 'Konzert, Teil 2', location: 'Kirche; Karlsruhe'})]));
+		expect(lines).toContain('SUMMARY:Konzert\\, Teil 2');
+		expect(lines).toContain('LOCATION:Kirche\\; Karlsruhe');
+	});
+
+	it('does not escape the URL, but keeps it on one line', async () => {
+		const lines = unfold(await convertToIcal([event({url: 'https://nachklang.art/konzert?a=1,2'})]));
+		expect(lines).toContain('URL:https://nachklang.art/konzert?a=1,2');
+	});
+
+	it('still writes the yearly rule for birthdays', async () => {
+		const lines = unfold(await convertToIcal([event({repeatFrequency: 'YEARLY', wholeDay: true})]));
+		expect(lines).toContain('RRULE:FREQ=YEARLY');
+	});
+
+	it('leaves the event\'s own dates untouched', async () => {
+		// formatDate used to add the whole-day end's extra day to the caller's Date in place.
+		const whole = event({
+			wholeDay: true,
+			startDateTime: new Date('2026-10-03T00:00:00'),
+			endDateTime: new Date('2026-10-03T23:59:00')
+		});
+		const endBefore = whole.endDateTime.getTime();
+		await convertToIcal([whole]);
+		expect(whole.endDateTime.getTime()).toBe(endBefore);
+	});
+});