Add Google login

[ICubE01]

Summary

• Users can now sign in with Google via OAuth 2.0 / OIDC
• New OAuth callback and post-login name-setup pages added to the frontend
• user_identities table added to link social accounts; password column made nullable for social-only users
• Renamed env var CORS_ALLOWED_ORIGIN → WEB_ORIGIN

Backend

Features

• SocialAuthController — OAuth flow endpoints: redirect URL, callback, account linking
• SocialAuthService — orchestrates find-or-create user logic for social login
• UserIdentityService — manages user ↔ provider identity mapping
• GoogleOidcProviderClient — Google OIDC token verification and user info fetch
• OidcProviderClientRegistry — registry for OIDC provider clients
• OidcStateStore — CSRF state parameter storage for OAuth flow
• UserController, UserService — PATCH /users/{id} endpoint to set display name
• SecurityConfig — open social login endpoints
• SocialAuthConfig — OIDC config bean

Chore

• V5__add_timestamp_columns.sql — add created_at, updated_at to users
• V6__modify_password_nullable.sql — make password nullable
• V7__add_user_identities_table.sql — new table for social identity linking
• Rename CORS_ALLOWED_ORIGIN → WEB_ORIGIN in .env.example, application.yaml, CI workflow

Frontend

Features

• LoginPage.tsx — Google sign-in button with divider
• GoogleCallbackPage.tsx — handles OAuth redirect, exchanges code, redirects on success
• InitNamePage.tsx — post-social-login page to set display name (skippable)
• App.tsx — new routes for /auth/callback/google and /init-name
• useAuth.tsx — add reloadUser helper to refresh user state after name update