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/lib/server/db/index.ts

@@ -1,15 +1,15 @@
 import Database from 'better-sqlite3';
 import { mkdirSync } from 'node:fs';
 import { dirname } from 'node:path';
+import { DATABASE_PATH, IMAGE_DIR } from '$lib/server/paths';
 import { runMigrations } from './migrate';
 
 let instance: Database.Database | null = null;
 
-export function getDb(path = process.env.DATABASE_PATH ?? './data/kochwas.db'): Database.Database {
+export function getDb(path = DATABASE_PATH): Database.Database {
   if (instance) return instance;
   mkdirSync(dirname(path), { recursive: true });
-  const imageDir = process.env.IMAGE_DIR ?? './data/images';
-  mkdirSync(imageDir, { recursive: true });
+  mkdirSync(IMAGE_DIR, { recursive: true });
   instance = new Database(path);
   instance.pragma('journal_mode = WAL');
   instance.pragma('foreign_keys = ON');

src/lib/server/paths.ts

@@ -0,0 +1,6 @@
+// Filesystem paths read from env at module load. Centralized so a misset
+// env var only causes one place to be wrong, not six. Both defaults match
+// the docker-compose volume mounts under `/app/data`.
+
+export const DATABASE_PATH = process.env.DATABASE_PATH ?? './data/kochwas.db';
+export const IMAGE_DIR = process.env.IMAGE_DIR ?? './data/images';