fix(auth): B-2.2 review fixes — Tailwind path + WCAG + race + flash + tests

C-1: Add templates/register.html (and templates/auth/**) to tailwind.config.js
content array so utility classes used by the signup template don't get purged
on next build. Rebuilt static/css/marketing.css; verified text-brand-navy/90
and min-h-[calc(100vh-62px)] are now compiled.

I-1: Replace flash() calls for missing required consents with WTForms
field-level errors (form.consent_cgu.errors.append / form.consent_confidentialite
.errors.append). Errors render inline next to each consent checkbox via
{% if form.consent_cgu.errors %}<p role="alert">…</p>{% endif %}. Prevents
session-backed flash messages from leaking across unrelated navigations.

I-2: Wrap user creation + flush in IntegrityError retry loop (max 5 attempts);
import IntegrityError from sqlalchemy.exc. Absorbs the inherent race between
_generate_unique_username's lookup and the subsequent flush under concurrent
signups. Added docstring note to _generate_unique_username explaining the
wrapper.

I-3: Move db.create_all() inside the try/finally in
test_signup_route_csrf_enforced so WTF_CSRF_ENABLED is restored even if
table creation fails.

I-4: Pin test_signup_rejects_duplicate_email assertion to status_code == 200
(WTForms validate_email raises ValidationError → form fails validation →
fall-through to default 200 render_template).

I-5: Add id="password-help" to the password help paragraph and
aria-describedby="password-help" to the password input so screen readers
announce the password requirements when the field is focused.

I-6: Bump flash banner text colors from -700/-800 to -900 variants
(text-amber-900, text-blue-900, text-red-900, text-green-900) for safer
WCAG 2.2 AA contrast against the -50 backgrounds. Same bump applied to the
new consent and password inline error renders.

static/css/tailwind.config.js

@@ -1,6 +1,6 @@
 /** @type {import('tailwindcss').Config} */
 module.exports = {
-  content: ['./templates/marketing/**/*.html', './templates/legal/**/*.html', './templates/billing/**/*.html', './templates/macros/**/*.html', './src/marketing/**/*.py', './src/legal/**/*.py', './src/billing/**/*.py'],
+  content: ['./templates/marketing/**/*.html', './templates/legal/**/*.html', './templates/billing/**/*.html', './templates/macros/**/*.html', './templates/auth/**/*.html', './templates/register.html', './src/marketing/**/*.py', './src/legal/**/*.py', './src/billing/**/*.py'],
   darkMode: 'class',
   theme: {
     extend: {