fix(mock-data): Fake-Daten bei leerer DB entfernt — ISMS 0%, Dashboard keine simulierten Trends, Compliance-Hub keine Fallback-Zahlen

- ISMS Overview: 14% → 0% bei leerer DB, "not_started" Status, alle Kapitel 0%
- Dashboard: 12-Monate simulierte Trend-Historie entfernt
- Compliance-Hub: Hardcoded Fallback-Statistiken (474/180/95/120/79/44/558/19) → 0
- SQLAlchemy Bug: `is not None` → `.isnot(None)` in SoA-Query
- Hardcoded chapter_7/8_status="pass" → berechnet aus Findings

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

backend-compliance/compliance/api/schemas.py

@@ -1677,7 +1677,7 @@ class ISO27001ChapterStatus(BaseModel):
 
 class ISO27001OverviewResponse(BaseModel):
     """Complete ISO 27001 status overview."""
-    overall_status: str  # "ready", "at_risk", "not_ready"
+    overall_status: str  # "ready", "at_risk", "not_ready", "not_started"
     certification_readiness: float  # 0-100
     chapters: List[ISO27001ChapterStatus]
     scope_approved: bool