[Feature] OAuth infrastructure setup

[fregataa]

Summary

Set up the common infrastructure for OAuth authentication to support multiple providers.

Scope

Backend (API Server)

• Add OAuth configuration structure in config
• Create OAuth callback handler base
• Implement user linking logic (OAuth account ↔ existing user)
• Add oauth_providers table or extend users table
• Handle first-time OAuth login (auto-registration)
• Handle returning OAuth user login

Frontend

• Create OAuth button components
• Add OAuth callback page (/auth/callback/[provider])
• Handle OAuth redirect flow
• Update auth store for OAuth

Database Schema

CREATE TABLE oauth_accounts (
    id UUID PRIMARY KEY,
    user_id UUID REFERENCES users(id),
    provider VARCHAR(50) NOT NULL,  -- google, kakao, naver, apple, facebook
    provider_user_id VARCHAR(255) NOT NULL,
    email VARCHAR(255),
    created_at TIMESTAMP DEFAULT NOW(),
    UNIQUE(provider, provider_user_id)
);

Acceptance Criteria

• OAuth base infrastructure implemented
• Database migration created
• Easy to add new OAuth providers
• Existing email/password login still works

Related Issues

• Blocks: Google, Kakao, Naver, Apple, Facebook OAuth issues