feat: add Terms & Conditions checkbox to signup (refs #4)

- Add terms_agreements table for legal audit trail
- Create terms-agreement feature capsule with repository
- Modify signup to create terms agreement atomically
- Add checkbox with PDF link to SignupForm
- Capture IP, User-Agent, terms version, content hash
- Update CLAUDE.md documentation index

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/auth/components/SignupForm.tsx

@@ -18,6 +18,9 @@ const signupSchema = z
       .regex(/[A-Z]/, 'Password must contain at least one uppercase letter')
       .regex(/[0-9]/, 'Password must contain at least one number'),
     confirmPassword: z.string(),
+    termsAccepted: z.literal(true, {
+      errorMap: () => ({ message: 'You must agree to the Terms & Conditions to create an account' }),
+    }),
   })
   .refine((data) => data.password === data.confirmPassword, {
     message: 'Passwords do not match',
@@ -42,8 +45,8 @@ export const SignupForm: React.FC<SignupFormProps> = ({ onSubmit, loading }) =>
   });
 
   const handleFormSubmit = (data: SignupRequest & { confirmPassword: string }) => {
-    const { email, password } = data;
-    onSubmit({ email, password });
+    const { email, password, termsAccepted } = data;
+    onSubmit({ email, password, termsAccepted });
   };
 
   return (
@@ -138,6 +141,31 @@ export const SignupForm: React.FC<SignupFormProps> = ({ onSubmit, loading }) =>
         )}
       </div>
 
+      <div className="flex items-start min-h-[44px]">
+        <label className="flex items-start cursor-pointer">
+          <input
+            {...register('termsAccepted')}
+            type="checkbox"
+            className="w-5 h-5 mt-0.5 rounded border-silverstone text-primary-600 focus:ring-abudhabi dark:border-silverstone dark:focus:ring-abudhabi"
+            aria-label="I agree to the Terms and Conditions"
+          />
+          <span className="ml-2 text-sm text-avus">
+            I agree to the{' '}
+            <a
+              href="/docs/v2026-01-03.pdf"
+              target="_blank"
+              rel="noopener noreferrer"
+              className="text-abudhabi hover:underline"
+            >
+              Terms & Conditions
+            </a>
+          </span>
+        </label>
+      </div>
+      {errors.termsAccepted && (
+        <p className="text-sm text-red-400">{errors.termsAccepted.message}</p>
+      )}
+
       <div className="pt-4">
         <Button type="submit" loading={loading} className="w-full min-h-[44px]">
           Create Account

frontend/src/features/auth/types/auth.types.ts

@@ -5,6 +5,7 @@
 export interface SignupRequest {
   email: string;
   password: string;
+  termsAccepted: boolean;
 }
 
 export interface SignupResponse {