Phase 0 Complete: Tasks 0.7-0.20

- 0.7: UI-Design-Richtlinien (docs/ui-design-guidelines.md, 535 lines)
- 0.8: Theme-Customization Backend (4 theme fields, migration 0023)
- 0.9: Theme-Customization Frontend (SettingsTheme.tsx, themeStore.ts, live preview)
- 0.10: RBAC-Audit (4 plugins secured, 53 routes with require_permission)
- 0.11: LiteLLM-Cleanup (llm_client.py migrated from httpx to litellm)
- 0.12: KI-Agent-Framework docs (plugin-development-guide.md, agent_capabilities field)
- 0.13: Heartbeat configurable (ProactiveSettings, migration 0024, frontend UI)
- 0.14: Unified Search Field-Level RBAC (resolve_permissions + filter_fields_by_permission)
- 0.15: Undo/History-System (EntityHistory model, service, routes, migration 0025, HistoryViewer)
- 0.16: Storage Backend (LocalStorage + S3Storage, DMS/attachments/mail updated)
- 0.17: Import/Export unified Contact fields (firstname, surname, email_1, phone_1)
- 0.18: .gitignore & Config-Cleanup (webui→frontend, python-jose removed, .env untracked)
- 0.19: Mail-Salt Security-Fix (per-account random salt, migration 0026)
- 0.20: AGPL replaced (PyMuPDF→pypdf, OnlyOffice→Collabora, LICENSE + THIRD_PARTY_LICENSES.md)

app/models/entity_history.py

@@ -0,0 +1,40 @@
+"""EntityHistory model — snapshot history for undo/restore functionality."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+from typing import Any
+
+from sqlalchemy import DateTime, ForeignKey, String, func
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.db import Base, TenantMixin
+
+
+class EntityHistory(Base, TenantMixin):
+    """Snapshot history for undo/restore functionality.
+
+    Every CRUD action (create/update/delete) stores a full entity snapshot
+    so users can undo changes or revert to previous versions.
+    """
+
+    __tablename__ = "entity_history"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        PGUUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True, index=True
+    )
+    entity_type: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    entity_id: Mapped[uuid.UUID] = mapped_column(PGUUID(as_uuid=True), nullable=False, index=True)
+    action: Mapped[str] = mapped_column(String(20), nullable=False)  # 'create', 'update', 'delete'
+    snapshot_before: Mapped[dict[str, Any] | None] = mapped_column(JSONB, nullable=True)
+    snapshot_after: Mapped[dict[str, Any] | None] = mapped_column(JSONB, nullable=True)
+    changes: Mapped[dict[str, Any] | None] = mapped_column(JSONB, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(), index=True
+    )