fix(backend): security scanner KeyError on miniKanren port findings

The convert_kanren_findings/1 function accessed finding.message and
finding.severity on all findings, but proto_findings from the kanren
engine only have :port, :protocol, :risk, :source (no :message or
:severity). Now uses Map.get with fallbacks for both finding shapes.

Also verified full backend stack:
- PostgreSQL via Podman container (postgres:17-alpine)
- Ecto migrations run (stacks, users, user_settings tables)
- DbStore auto-fallback working (NativeBridge.available?/0)
- POST /api/stacks creates + persists to PostgreSQL
- POST /api/stacks/:id/validate returns real findings (12 checks)
- POST /api/stacks/:id/security-scan returns real vulnerabilities
- POST /api/stacks/:id/gap-analysis returns 8 real gaps with fixes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

backend/lib/stapeln/security_scanner.ex

@@ -474,10 +474,14 @@ defmodule Stapeln.SecurityScanner do
 
   defp convert_kanren_findings(port_findings) do
     Enum.map(port_findings, fn finding ->
+      # Handle both db_findings (%{message, severity}) and proto_findings (%{protocol, risk})
+      message = Map.get(finding, :message, "Port #{finding.port} (#{Map.get(finding, :protocol, "unknown")})")
+      severity = Map.get(finding, :severity, Map.get(finding, :risk, :info))
+
       %{
-        title: finding.message,
-        severity: Atom.to_string(finding.severity),
-        description: "miniKanren reasoning engine flagged port #{finding.port}: #{finding.message}",
+        title: message,
+        severity: if(is_atom(severity), do: Atom.to_string(severity), else: severity),
+        description: "miniKanren reasoning engine flagged port #{finding.port}: #{message}",
         affectedComponent: "port:#{finding.port}",
         cveId: nil,
         fixAvailable: false,