feat(audit): add comprehensive audit logging with SIEM integration

- Add AuditLog model with append-only design (timestamp, user, action, resource, IP, details, severity)
- Add audit_service.py with record/query helpers and SIEM forwarding (Syslog RFC 5424, HTTP/webhook)
- Add /api/audit-logs REST endpoints with filtering and pagination
- Add /admin/audit-logs viewer UI with real-time filters
- Add SIEM config settings (syslog, HTTP for Splunk HEC/Logstash/Grafana Loki)
- Add Alembic migration 027_add_audit_logs
- Add navigation link in admin menu
- Add comprehensive tests (20 tests covering model, service, SIEM, API, view)

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/models.py

@@ -146,6 +146,27 @@ class SettingsAuditLog(Base):
     action = Column(String, nullable=False)  # "update" or "delete"
 
 
+class AuditLog(Base):
+    """Comprehensive audit log for compliance tracking.
+
+    Records all significant actions: login/logout, document CRUD, settings
+    changes, and administrative operations.  Rows are append-only; the API
+    and service layer never update or delete entries.
+    """
+
+    __tablename__ = "audit_logs"
+
+    id = Column(Integer, primary_key=True, index=True)
+    timestamp = Column(DateTime(timezone=True), server_default=func.now(), nullable=False, index=True)
+    user = Column(String, nullable=False, index=True)  # Username or "anonymous" / "system"
+    action = Column(String, nullable=False, index=True)  # e.g. "login", "document.create", "settings.update"
+    resource_type = Column(String, nullable=True, index=True)  # e.g. "document", "user", "settings"
+    resource_id = Column(String, nullable=True)  # ID of the affected resource
+    ip_address = Column(String, nullable=True)  # Client IP address
+    details = Column(Text, nullable=True)  # JSON-encoded extra context
+    severity = Column(String(16), nullable=False, server_default="info")  # info / warning / error / critical
+
+
 class SavedSearch(Base):
     """User-defined saved search filters for quick access to frequently used filter combinations."""