refactor(server): IMAGE_DIR/DATABASE_PATH zentralisieren + Doku-Drift fixen

src/lib/server/paths.ts: zentrale Auflösung der env-vars; vorher 6×
IMAGE_DIR und 2× DATABASE_PATH dupliziert mit identischen Defaults.

Migrierte Sites:
- src/lib/server/db/index.ts (DATABASE_PATH + IMAGE_DIR)
- src/routes/api/admin/backup/+server.ts
- src/routes/api/domains/+server.ts
- src/routes/api/domains/[id]/+server.ts
- src/routes/api/recipes/import/+server.ts
- src/routes/api/recipes/[id]/image/+server.ts
- src/routes/images/[filename]/+server.ts

ARCHITECTURE.md:
- 49 Flachwitze -> 150 (waren tatsaechlich 150)
- 'search/' Route entfernt — wurde nie als eigene Route gebaut, Suche
  laeuft direkt auf der Homepage via API-Calls

Findings aus zweiter Review-Runde (siehe OPEN-ISSUES-NEXT.md)

src/routes/api/admin/backup/+server.ts

@@ -1,12 +1,10 @@
 import type { RequestHandler } from './$types';
 import { createBackupStream, backupFilename } from '$lib/server/backup/export';
+import { DATABASE_PATH, IMAGE_DIR } from '$lib/server/paths';
 import { Readable } from 'node:stream';
 
-const DB_PATH = process.env.DATABASE_PATH ?? './data/kochwas.db';
-const IMAGE_DIR = process.env.IMAGE_DIR ?? './data/images';
-
 export const GET: RequestHandler = async () => {
-  const archive = createBackupStream({ dbPath: DB_PATH, imagesDir: IMAGE_DIR });
+  const archive = createBackupStream({ dbPath: DATABASE_PATH, imagesDir: IMAGE_DIR });
   const filename = backupFilename();
   return new Response(Readable.toWeb(archive) as ReadableStream, {
     status: 200,