feat(auth): add local user signup, email verification, and Stripe billing

- Add LocalUser model with bcrypt password hashing, email verification
  tokens, and password reset tokens
- Add ALLOW_LOCAL_SIGNUP config flag (requires SMTP to be configured)
- Add Stripe billing config fields (STRIPE_SECRET_KEY, etc.)
- Add stripe_customer_id to UserProfile and stripe_price_id_monthly/
  stripe_price_id_yearly to SubscriptionPlan
- Create migration 018_add_local_users_and_billing
- Add app/utils/local_auth.py: hash_password, verify_password,
  generate_token, is_token_expired, send_verification_email,
  send_password_reset_email, build_session_user
- Add app/api/local_auth.py: signup, email verification, password reset
  endpoints plus signup/verify-email-sent/reset-password page routes
- Add app/api/billing.py: Stripe Checkout, Customer Portal, and webhook
  endpoints; syncs subscription tier from webhook events
- Update auth() to check LocalUser table before admin credentials fallback
- Update login() to pass allow_signup context variable to template
- Add signup.html, verify_email_sent.html, password_reset_form.html,
  billing_success.html templates (Alpine.js, Tailwind, WCAG 2.1 AA)
- Update login.html to show 'Create account' link when signup enabled
- Update pricing.html CTA buttons to use Stripe Checkout for paid tiers
- Add docs/BillingSetup.md with setup guide, webhook config, compliance
- Add tests/test_local_auth.py (42 tests) and tests/test_billing.py
  (31 tests); all 106 tests in the modified test suite pass
- Add stripe>=7.0.0,<15.0.0 to requirements.txt

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/config.py

@@ -172,6 +172,23 @@ class Settings(BaseSettings):
     authentik_config_url: Optional[str] = None
     oauth_provider_name: Optional[str] = None  # Name to display for the OAuth provider
 
+    # Local user signup
+    allow_local_signup: bool = Field(
+        default=False,
+        description=(
+            "Allow users to self-register with email and password. "
+            "Requires email (SMTP) to be configured for verification emails. "
+            "Default: False (registration disabled, admin creates users)."
+        ),
+    )
+
+    # Stripe billing
+    stripe_secret_key: Optional[str] = None
+    stripe_publishable_key: Optional[str] = None
+    stripe_webhook_secret: Optional[str] = None
+    stripe_success_url: Optional[str] = None  # e.g. https://app.example.com/billing/success
+    stripe_cancel_url: Optional[str] = None  # e.g. https://app.example.com/pricing
+
     # IMAP 1
     imap1_host: Optional[str] = None
     imap1_port: Optional[int] = 993