feat: add API-based OAuth endpoints for webapp integration

[TooAngel]

Summary

• Add GET /api/auth/url?redirect_uri=<callback> - returns GitHub OAuth URL
• Add POST /api/auth/callback - exchanges OAuth code for session, returns sessionId
• Update integration pattern docs to describe API-based flow

This enables the webapp to handle the OAuth UX directly:

1. Webapp calls /api/auth/url to get the GitHub OAuth URL
2. Webapp redirects user to GitHub
3. GitHub redirects back to webapp callback with code
4. Webapp calls /api/auth/callback with code, receives sessionId
5. Webapp sets sessionId as httpOnly cookie

Test plan

• Test /api/auth/url returns valid OAuth URL with correct redirect_uri
• Test /api/auth/callback exchanges code and returns sessionId
• Test full OAuth flow with webapp integration

[worlddriven]

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🗓️ Merge Date: 2025-12-25 at 01:12:49 UTC (today)
📅 Started: 2025-12-23 at 06:15:59 UTC
⚡ Speed Factor: 0.18 (82% faster due to reviews)
✅ Positive votes: 312/380 contribution weight (coefficient: 0.82)
📈 Base Merge Time: 10 days → Current: 2 days

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📋 Recent Activity

• 2025-12-23, 06:16:07 - Pull request opened
• 2025-12-23, 06:16:08 - Pull request opened
• 2025-12-25, 01:51:18 - Pull request merged by worlddriven ✅

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This comment is automatically updated by worlddriven