feat(domains): Favicons laden und im Filter anzeigen

Für jede Whitelist-Domain wird das Favicon jetzt einmalig geladen und
im image-Verzeichnis abgelegt. SearchFilter zeigt das Icon neben dem
Domain-Namen im Filter-Dropdown.

- Migration 009: allowed_domain.favicon_path (NULL = noch nicht geladen).
- Neues Modul $lib/server/domains/favicons.ts:
  fetchAndStoreFavicon(domain, imageDir) + ensureFavicons(db, imageDir)
  für Bulk-Nachzug; 8 parallele Worker mit 3s-Timeout.
- Reihenfolge: erst /favicon.ico der Domain, Fallback Google-Service.
- GET /api/domains zieht fehlende Favicons auf Abruf nach;
  POST /api/domains lädt direkt im selben Call.
- .ico + .svg jetzt in der /images/[filename]-Route erlaubt.

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

tests/integration/favicons.test.ts

@@ -0,0 +1,35 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mkdtempSync, rmSync, readdirSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { openInMemoryForTest } from '../../src/lib/server/db';
+import { addDomain, listDomains } from '../../src/lib/server/domains/repository';
+import { ensureFavicons } from '../../src/lib/server/domains/favicons';
+
+let imageDir: string;
+
+beforeEach(() => {
+  imageDir = mkdtempSync(join(tmpdir(), 'kochwas-favicon-'));
+});
+
+afterEach(() => {
+  rmSync(imageDir, { recursive: true, force: true });
+});
+
+describe('ensureFavicons', () => {
+  it('is a no-op when every domain already has a favicon_path', async () => {
+    const db = openInMemoryForTest();
+    const d = addDomain(db, 'example.com');
+    // simulate already-stored favicon
+    db.prepare('UPDATE allowed_domain SET favicon_path = ? WHERE id = ?').run(
+      'favicon-abc.png',
+      d.id
+    );
+    await ensureFavicons(db, imageDir);
+    // No file written, no DB state changed
+    expect(readdirSync(imageDir).length).toBe(0);
+    const domains = listDomains(db);
+    expect(domains[0].favicon_path).toBe('favicon-abc.png');
+  });
+
+});