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/config.py

@@ -849,6 +849,49 @@ class Settings(BaseSettings):
         ),
     )
 
+    # SIEM / External Audit Log Forwarding
+    # Forward audit events to external SIEM systems for centralised monitoring.
+    audit_siem_enabled: bool = Field(
+        default=False,
+        description="Enable forwarding of audit events to an external SIEM system.",
+    )
+    audit_siem_transport: str = Field(
+        default="syslog",
+        description=(
+            "Transport used to forward audit events. "
+            "Options: 'syslog' (RFC 5424 over UDP/TCP), 'http' (JSON POST to a webhook URL, "
+            "compatible with Splunk HEC, Logstash HTTP input, Grafana Loki, etc.)."
+        ),
+    )
+    audit_siem_syslog_host: str = Field(
+        default="localhost",
+        description="Hostname or IP of the syslog receiver.",
+    )
+    audit_siem_syslog_port: int = Field(
+        default=514,
+        description="Port of the syslog receiver.",
+    )
+    audit_siem_syslog_protocol: str = Field(
+        default="udp",
+        description="Protocol for syslog transport: 'udp' or 'tcp'.",
+    )
+    audit_siem_http_url: str = Field(
+        default="",
+        description=(
+            "HTTP endpoint URL for SIEM webhook delivery. "
+            "Supports Splunk HEC (https://splunk:8088/services/collector/event), "
+            "Logstash HTTP input, Grafana Loki push API, or any JSON-accepting endpoint."
+        ),
+    )
+    audit_siem_http_token: str = Field(
+        default="",
+        description="Bearer / HEC token included in the Authorization header of SIEM HTTP requests.",
+    )
+    audit_siem_http_custom_headers: str = Field(
+        default="",
+        description="Comma-separated 'Key:Value' pairs of extra headers for SIEM HTTP requests.",
+    )
+
     # UI / Appearance
     ui_default_color_scheme: str = Field(
         default="system",