Move auth check to frontend for CDN-friendly caching

[zhangzhenghao]

Summary

This PR moves authentication check from backend to frontend for CDN-friendly caching, following the same pattern as gitrec PR #166.

Backend Changes

1. Remove login check from dashboard handler

   • The dashboard function now serves static files directly without checking login status
   • Frontend handles authentication check before rendering protected pages

2. Add /api/me endpoint

   • Returns {is_authenticated: true/false, user_info: {...}}
   • No login required - frontend uses this to check auth state
   • Works with both OIDC and basic auth modes

3. Add CacheControlFilter

   • Sets Cache-Control: private, no-store, no-cache, must-revalidate for all API endpoints
   • Prevents CDN from caching dynamic API responses

Benefits

• CDN-friendly: Dashboard pages return static files that can be safely cached by CDN edge nodes
• Reduced backend load: Static file serving can be handled by CDN
• Better performance: Users get faster page loads from CDN cache
• Security: API endpoints explicitly disable caching to protect user data

Frontend Changes (Required)

Frontend repository (gorse-io/dashboard) needs to be updated separately to:

1. Add router guard that calls /api/me before accessing protected routes
2. Redirect to /login if is_authenticated is false

Testing

# Build and verify
go build ./master/

# Test /api/me endpoint
curl http://localhost:8088/api/me
# Expected: {"is_authenticated": false, "user_info": null}

# After login
curl http://localhost:8088/api/me --cookie "session=..."
# Expected: {"is_authenticated": true, "user_info": {"name": "admin"}}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.96%. Comparing base (2241300) to head (c4b6bcb).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1253      +/-   ##
==========================================
- Coverage   73.01%   72.96%   -0.05%     
==========================================
  Files          91       91              
  Lines       16694    16734      +40     
==========================================
+ Hits        12189    12210      +21     
- Misses       3262     3280      +18     
- Partials     1243     1244       +1     

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