feat(auth): B-2.4 OAuth Microsoft/Google + magic link (Loi 25 deferred consent)

Adds Microsoft 365 + Google OAuth providers (separate from the existing
generic OIDC SSO at src/auth/sso.py) and a passwordless magic-link login
flow. New OAuth signups capture Loi 25 art. 14 consents (4 granular
checkboxes) BEFORE creating the User row via /auth/oauth/finish-signup.

Per compatibility-audit.md C2:
- No src/auth_extended/ directory — extends src/auth/ in place
- No new User columns — reuses sso_provider/sso_subject + email_verified
- Magic-link tokens via itsdangerous URLSafeTimedSerializer (15-min, no DB)
- All routes added to existing auth_bp; templates extend marketing/base.html
- Anti-enumeration on /auth/magic-link (generic flash for unknown OR
  unverified emails) and /auth/magic-link/<token> (same flash for
  invalid/expired/unverified-user)

Files added:
- src/auth/oauth_providers.py — Microsoft + Google OAuth registration,
  is_oauth_provider_enabled(), find_user_by_oauth(), create_oauth_user_with_consent()
- src/auth/magic_link.py — generate/consume magic-link tokens
- templates/auth/magic_link_request.html, templates/auth/oauth_finish_signup.html
- tests/test_oauth_magic_link.py + tests/_run_oauth_magic_link_windows.py (16 tests)
- config/env.oauth.example

Files modified:
- src/api/auth.py — 5 new routes (oauth_provider_login/callback,
  oauth_finish_signup, magic_link_request/consume); login flashes translated FR;
  oauth_*_enabled flags passed to login template
- src/app.py — wires init_oauth_providers(app) after blueprint registration
- src/services/email.py — adds send_magic_link_email() (FR + DictIA brand)
- templates/login.html — refondu IN PLACE (was 178 lines legacy Vue/TW3)
  to extend marketing/base.html with OAuth buttons, password form,
  magic-link CTA, signup link
- templates/auth/check_email.html — adds action='magic_link' branch
- static/css/tailwind.config.js — adds templates/login.html to content
- static/css/marketing.css — rebuilt

Tests: 16/16 PASS via Windows manual driver.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/css/marketing.css

@@ -546,6 +546,9 @@
   .my-2 {
     margin-block: calc(var(--spacing) * 2);
   }
+  .my-3 {
+    margin-block: calc(var(--spacing) * 3);
+  }
   .my-4 {
     margin-block: calc(var(--spacing) * 4);
   }
@@ -1443,9 +1446,6 @@
   .border-\[var\(--border-accent\)\] {
     border-color: var(--border-accent);
   }
-  .border-\[var\(--border-danger\)\] {
-    border-color: var(--border-danger);
-  }
   .border-\[var\(--border-focus\)\] {
     border-color: var(--border-focus);
   }