fix(security): resolve Snyk Code findings — Tar Slip, Path Traversal, Open Redirect, XSS

Tar Slip (CWE-22) — 3 import services:
  threat_actor, lolbas, caldera: add path validation before extractall()
  to prevent malicious zip members with ../ escaping the target directory.
  (sigma, elastic, atomic already had this protection)

Path Traversal (CWE-23) — professional_reports.py:
  Add _assert_safe_report_path() check on all 5 report endpoints to
  verify the generated filepath stays within REPORT_OUTPUT_DIR.

Open Redirect (CWE-601) — sso.py:
  Validate IdP redirect URL scheme (must be http/https) before
  issuing RedirectResponse, blocking javascript: and data: redirects.

DOM XSS (CWE-79) — 4 frontend pages:
  Create src/utils/url.ts with safeUrl() that rejects non-http/https
  protocols; apply to actor.mitre_url, ref.url, intel.url.
  Sanitize framework name to alphanumeric-only before DOM insertion.
  Restrict evidence MIME types to an explicit safe allowlist (png/jpg/gif/webp).

Hardcoded credentials (CWE-798):
  verify_gaps.py, create_wiki.py: replace literal passwords with
  environment variable reads (AEGIS_ADMIN_PASSWORD, GITEA_PASSWORD).

frontend/src/pages/TechniqueDetailPage.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { useParams, useNavigate } from "react-router-dom";
 import MarkdownText from "../components/MarkdownText";
+import { safeUrl } from "../utils/url";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import {
   Loader2,
@@ -640,7 +641,7 @@ export default function TechniqueDetailPage() {
                   </p>
                 </div>
                 <a
-                  href={intel.url}
+                  href={safeUrl(intel.url)}
                   target="_blank"
                   rel="noopener noreferrer"
                   className="flex items-center gap-1 text-sm text-cyan-400 hover:underline"