refactor(api): requireAuth returns resolved Principal (closes #194)

[cristim]

Summary

• requireAuth now returns (*Principal, error) instead of bare error by delegating to the new authenticatePrincipal helper (already added in this branch)
• authenticatePrincipal cleaned up: nil-auth guard moved before the API-key path (fail-closed per feedback_fail_closed_middleware.md), dead userIface/userFields locals removed
• Router updated to _, err := r.h.requireAuth(...) to handle the new return shape
• Existing TestRequireAuth_* tests upgraded to assert the returned Principal (kind, role, UserID, Session pointer) for each credential type

Test plan

• go build ./... clean
• go test github.com/LeanerCloud/CUDly/internal/api/... github.com/LeanerCloud/CUDly/internal/auth/... — 1852 passed
• TestRequireAuth_AdminAPIKey now asserts PrincipalAdminAPIKey / role=admin
• TestRequireAuth_UserSession now asserts PrincipalSession, UserID, Role, Session pointer
• TestRequireAuth_NoCredential_Rejects still asserts 401 ClientError

Closes #194

[coderabbitai]

Warning

Review limit reached

@cristim, we couldn't start this review because you've reached your PR review rate limit.

More reviews will be available in 25 minutes and 21 seconds. Learn how PR review limits work.

Your organization has run out of usage credits. Purchase more in the billing tab.

⌛ How to resolve this issue?

After more reviews become available, a review can be triggered using the @coderabbitai review command as a PR comment. Alternatively, push new commits to this PR.

We recommend that you space out your commits to avoid hitting the rate limit.

🚦 How do rate limits work?

CodeRabbit enforces hourly rate limits for each developer per organization.

Our paid plans include higher PR review limits than trial, open-source, and free plans. In all cases, reviews become available again over time. During sustained high-volume PR review activity, CodeRabbit may temporarily slow when the next review becomes available.

Please see our Fair Usage Limits Policy for further information.

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ff6ef979-af83-4408-80c2-101e4bf7c837

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and 74d8767.

📒 Files selected for processing (3)

• internal/api/middleware.go
• internal/api/router.go
• internal/api/router_authuser_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/194-wave18

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[cristim]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.