chore: remove MITRE integrated lab code and CSRF_user_tbl model

[ANIRUDH-SJ]

Summary

Removes all MITRE integrated lab code from the main application as part of the V3
cleanup (#395). Theory-only lesson pages (mitre_top1 through mitre_top25) are
preserved.

What was removed

Integrated lab functions from introduction/mitre.py:

• csrf_lab_login, csrf_transfer_monei, csrf_transfer_monei_api (CSRF lab —
  CWE-352)
• mitre_lab_25, mitre_lab_25_api (code injection lab — CWE-94)
• mitre_lab_17, mitre_lab_17_api (command injection lab — CWE-78)
• command_out helper function

Integrated lab templates:

• mitre/csrf_lab_login.html
• mitre/csrf_dashboard.html
• mitre/mitre_lab_25.html
• mitre/mitre_lab_17.html

Broken import and unused dependencies:

• Removed from .models import CSRF_user_tbl (model was already deleted)
• Removed unused imports: datetime, re, subprocess, hashlib, jwt,
  csrf_exempt, JsonResponse, HttpResponse, HttpResponseBadRequest, redirect

Database cleanup:

• Added migration 0023_remove_csrf_user_tbl to drop the CSRF_user_tbl table

What was kept

• All 25 MITRE theory lesson views (mitre_top1 through mitre_top25) and their
  templates — these are educational content pages, not interactive labs
• All MITRE lesson URL patterns in urls.py

Notes

• The CSRF lab (mitre/9), code injection lab (mitre/25), and command injection lab
  (mitre/17) do not currently have dockerized replacements. These should be tracked as
  separate issues if dockerized versions are needed.

Test plan

• Application starts without errors
• All 25 MITRE lesson pages load correctly (/mitre/1 through /mitre/25)
• No broken imports or references
• Migration runs cleanly: python manage.py migrate

Part of #395