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/models.py

@@ -172,6 +172,31 @@ class WebhookConfig(Base):
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
 
 
+class LocalUser(Base):
+    """A locally-registered user authenticated by email and bcrypt password.
+
+    Created during the self-registration flow when ``allow_local_signup`` is
+    enabled.  The account is inactive (``is_active=False``) until the user
+    clicks the verification link sent to their email address.
+    """
+
+    __tablename__ = "local_users"
+
+    id = Column(Integer, primary_key=True, index=True)
+    email = Column(String(255), unique=True, nullable=False, index=True)
+    username = Column(String(64), unique=True, nullable=False, index=True)
+    display_name = Column(String(255), nullable=True)
+    hashed_password = Column(String(255), nullable=False)
+    is_active = Column(Boolean, nullable=False, default=False, server_default="0")
+    is_admin = Column(Boolean, nullable=False, default=False, server_default="0")
+    email_verification_token = Column(String(128), nullable=True)
+    email_verification_sent_at = Column(DateTime(timezone=True), nullable=True)
+    password_reset_token = Column(String(128), nullable=True)
+    password_reset_sent_at = Column(DateTime(timezone=True), nullable=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+
 class UserProfile(Base):
     """Per-user profile for admin-managed settings in multi-user mode.
 
@@ -214,6 +239,7 @@ class UserProfile(Base):
     onboarding_completed_at = Column(DateTime(timezone=True), nullable=True)
     contact_email = Column(String(255), nullable=True)
     preferred_destination = Column(String(50), nullable=True)
+    stripe_customer_id = Column(String(64), nullable=True)
 
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
@@ -262,6 +288,8 @@ class SubscriptionPlan(Base):
     sort_order = Column(Integer, nullable=False, default=0)
     features = Column(Text, nullable=True)  # JSON-encoded list[str]
     api_access = Column(Boolean, nullable=False, default=False)
+    stripe_price_id_monthly = Column(String(64), nullable=True)
+    stripe_price_id_yearly = Column(String(64), nullable=True)
 
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())