TASK-033: Remove session token from JSON response body

Fixes a critical security vulnerability where the session token was
being returned in the JSON response body of POST /api/auth/login.
This exposed the token to JavaScript, allowing malicious scripts to
steal it and bypass the HttpOnly cookie protection.

Changes:
- Backend: Remove 'token' field from LoginResponse model (auth.py)
- Backend: Update login() endpoint to return only 'expires_at'
- Frontend: Update LoginResponse type to exclude 'token' field
- Backend: Update test helper _login() to extract token from cookie
- Backend: Update test cases to verify token is NOT in response body
- Documentation: Add section 'Authentication Endpoints' in Backend-Development.md
- Documentation: Update Web-Development.md to explain HttpOnly cookie benefits

Security benefit: Session tokens are now only accessible via HttpOnly
cookies, protected from JavaScript access, XSS attacks, and malicious
third-party scripts. The frontend continues to use only the cookie for
authentication.

All auth tests pass (23 tests). Type checking and linting pass with
zero errors.

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

frontend/src/types/auth.ts

@@ -9,7 +9,6 @@ export interface LoginRequest {
 
 /** Successful login response from the API. */
 export interface LoginResponse {
-  token: string;
   expires_at: string;
 }