fix: fix GitHub OAuth redirect URI and URL encoding [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Description

Fixes #821 — GitHub OAuth sign-in flow returns a 404 when users click "Sign in with GitHub."

Root Cause

Two issues caused the 404:

1. Redirect URI mismatch

The default GITHUB_REDIRECT_URI was set to http://localhost:3000/auth/callback, but the frontend React route is /auth/github/callback (see frontend/src/App.tsx). After GitHub authorization, the redirect hit the wrong route, returning a 404.

2. Missing URL encoding in authorize URL construction

The get_github_authorize_url function constructed the GitHub OAuth URL using raw string concatenation (f-string), which does not URL-encode parameter values. The scope parameter contains a space (read:user user:email) that must be encoded as %20 or + for GitHub to accept the URL. Other parameters like redirect_uri could also contain characters requiring encoding.

Changes

1. backend/app/services/auth_service.py:
   • Fixed GITHUB_REDIRECT_URI default from /auth/callback to /auth/github/callback to match the frontend route
   • Added from urllib.parse import urlencode import
   • Replaced raw string concatenation with urlencode(params) for proper URL parameter encoding

Verification

• Redirect URI now matches frontend route: /auth/github/callback
• Authorize URL parameters are properly URL-encoded
• No new imports beyond Python stdlib