fix: Restore all files lost during destructive rebase

A previous `git pull --rebase origin main` dropped 177 local commits,
losing 3400+ files across admin-v2, backend, studio-v2, website,
klausur-service, and many other services. The partial restore attempt
(660295e2) only recovered some files.

This commit restores all missing files from pre-rebase ref 98933f5e
while preserving post-rebase additions (night-scheduler, night-mode UI,
NightModeWidget dashboard integration).

Restored features include:
- AI Module Sidebar (FAB), OCR Labeling, OCR Compare
- GPU Dashboard, RAG Pipeline, Magic Help
- Klausur-Korrektur (8 files), Abitur-Archiv (5+ files)
- Companion, Zeugnisse-Crawler, Screen Flow
- Full backend, studio-v2, website, klausur-service
- All compliance SDKs, agent-core, voice-service
- CI/CD configs, documentation, scripts

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

backend/config.py

@@ -0,0 +1,18 @@
+from pathlib import Path
+
+BASE_DIR = Path.home() / "Arbeitsblaetter"
+EINGANG_DIR = BASE_DIR / "Eingang"
+BEREINIGT_DIR = BASE_DIR / "Bereinigt"
+EDITIERBAR_DIR = BASE_DIR / "Editierbar"
+NEU_GENERIERT_DIR = BASE_DIR / "Neu_generiert"
+
+VALID_SUFFIXES = {".jpg", ".jpeg", ".png", ".pdf", ".JPG", ".JPEG", ".PNG", ".PDF"}
+
+# Ordner sicherstellen
+for d in [EINGANG_DIR, BEREINIGT_DIR, EDITIERBAR_DIR, NEU_GENERIERT_DIR]:
+    d.mkdir(parents=True, exist_ok=True)
+
+
+def is_valid_input_file(path: Path) -> bool:
+    """Gemeinsame Filterlogik für Eingangsdateien."""
+    return path.is_file() and not path.name.startswith(".") and path.suffix in VALID_SUFFIXES