feat: user database schema OAuth fields + Google authentication

[Depo-dev]

Summary

• Issue Create User Database Schema #105 - User DB Schema: Extends the existing User model with OAuth support: password is now nullable (for OAuth-only accounts), adds an AuthProvider enum (LOCAL/GOOGLE), and provider/providerId fields for multi-provider design. The existing avatarUrl field already covers profileImage. firstName/lastName cover fullName. All other required fields (id, email, createdAt, updatedAt, unique email index) were already production-ready.
• Issue Google Authentication Implementation #104 - Google Auth: Implements full Google OAuth 2.0 sign-in/sign-up via passport-google-oauth20. New users are auto-created on first Google login; existing local accounts are linked if they share the same email. Returns a JWT identical to the local login flow.

Changes

File	What changed
prisma/schema.prisma	AuthProvider enum; password nullable; provider/providerId fields on User
src/auth/auth.service.ts	googleLogin() method - upsert logic for Google users
src/auth/strategies/google.strategy.ts	New - passport-google-oauth20 Passport strategy
src/auth/guards/google-auth.guard.ts	New - GoogleAuthGuard
src/auth/auth.controller.ts	GET /auth/google and GET /auth/google/callback endpoints
src/auth/auth.module.ts	Register GoogleStrategy as provider
package.json	Add passport-google-oauth20 + @types/passport-google-oauth20
.env.example	Document GOOGLE_CLIENT_ID, GOOGLE_CLIENT_SECRET, GOOGLE_CALLBACK_URL

Test plan

• npm install picks up passport-google-oauth20
• npx prisma migrate dev applies the schema changes without errors
• GET /auth/google redirects to Google consent screen
• Completing Google OAuth flow returns { access_token, user } from /auth/google/callback
• New Google users are created in DB with provider=GOOGLE
• Existing local user with matching email gets provider/providerId linked on Google login
• Local POST /auth/login still rejects accounts with no password (OAuth-only users)

Closes #104
Closes #105

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