Harden CSRF, ALLOWED_HOSTS, and social login security

[daiv-agent]

Security hardening across three fronts: ALLOWED_HOSTS now fails closed in production, CSRF enforcement on session-authenticated ninja routes is pinned by a regression test, and social (GitHub/GitLab) logins are POST-initiated to prevent login-CSRF.

Key Changes:

• common.py now defaults DJANGO_ALLOWED_HOSTS to the empty list (was "*"), so Django refuses to serve until the operator configures hosts — blocking host-header poisoning from reaching login-by-code emails and OAuth metadata. debug.py (dev only) and testing.py override to ["*"] so runserver and the test client (Host: testserver) keep working; docs/reference/env-variables.md documents the new default.
• allauth.py sets SOCIALACCOUNT_LOGIN_ON_GET = False (was True), and account/login.html renders the GitHub/GitLab provider links as POST forms with {% csrf_token %} instead of GET anchors, so a crafted GET link can no longer trigger a login.
• Added tests/unit_tests/chat/api/test_csrf.py pinning both halves of CSRF on POST /api/chat/cancel with Django's real Client(enforce_csrf_checks=True): a cookie-authenticated POST without X-CSRFToken is rejected (403), and one carrying a matching token reaches the handler (200).
• daiv/daiv/api.py documents that django-ninja 1.x enforces CSRF per-route at the auth-class level (django_auth/SessionAuth runs check_csrf), not via a NinjaAPI(csrf=...) flag; Bearer/API-key clients are unaffected.
• AGENTS.md updated with the ALLOWED_HOSTS fail-closed and ninja-CSRF invariants, plus the parable git-source pinning note ([tool.uv.sources] rev freezes the commit, not the dep line).

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