fix(auth): remove dead challenge/response surface (security findings 8+16)

[frdomovic]

Security finding

[M] Challenge/response has no replay protection / is decorative (security-review items 8 + 16) — crates/auth/src/auth/token/jwt.rs.

GET /auth/challenge mints a random nonce + jti, but nothing ever stores them and verify_challenge has zero callers — a signed challenge is never demanded by any login flow. The surface is security theater: it suggests proof-of-possession that doesn't exist, and any real fix (a single-use nonce store consumed by a wallet provider's token exchange) would be dead machinery, since core ships no wallet provider — plus a naively-persisted nonce per unauthenticated GET is a storage-write DoS vector.

Decision (team): no NEAR/wallet login this iteration — user_password is the only provider, and password auth needs no PoP step. Remove the dead surface instead of building a nonce store for a provider that doesn't exist.

Before → after

	Before	After
GET /auth/challenge	Public route returning a nonce+jti that nothing verifies (replayable for its whole 5-min "lifetime" — if anything had consumed it).	Route removed (404).
TokenManager	generate_challenge / verify_challenge (dead) + ChallengeClaims.	Removed.
Secrets	Three secret types incl. JwtChallenge, rotated forever for nothing.	JwtAuth + Csrf only. Legacy system:secrets:jwt_challenge* storage entries are simply never read again (doc-commented on SecretType).
Docs	operate/auth.mdx et al. describe challenge lifetime / wallet challenge auth.	Updated to the real surface.

Deliberately kept: the TEE/KMS attestation challenge flow (crates/merod/src/kms) and libp2p/sync uses of the word — unrelated subsystems. NodeChallenge DTOs in calimero-server-primitives are unrouted legacy public API — candidate for a separate dead-code sweep.

How to test

cargo test -p mero-auth   # 93 tests green
cargo check -p mero-auth -p calimero-server

Manual: curl http://<node>/auth/challenge → 404; user-password login and token refresh unaffected.

Client follow-ups (separate PRs, non-breaking today since no server ever advertises a wallet provider)

• mero-js: remove getChallenge() (src/auth-api/auth-client.ts:86), ChallengeResponse type, and the MSW mock — breaking API change for the package, needs a version bump.
• auth-frontend: remove the dead near_wallet challenge→sign→token branch in LoginView.tsx:277-340 + mocks/fixtures.

Related: #3079/#3080/#3081/#3082/#3083 (the auth security-review stack). Expect a trivial textual conflict with #3083 in jwt.rs/secrets.rs — whichever merges second rebases.

[meroreviewer]

🤖 MeroReviewer

Reviewed by 2 agents | Quality score: 90% | Review time: 190.4s

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✅ No Issues Found

All agents reviewed the code and found no issues. LGTM! 🎉

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_{🤖 Generated by MeroReviewer | Review ID: review-8292737c}

[meroreviewer]

Documentation Review

The following documentation may need updates based on the changes in this PR:

• 🟡 architecture/crates/auth.html: Files matching crates/auth/** were changed but architecture/crates/auth.html was not updated (per source_to_docs_mapping).

[meroreviewer]

🤖 AI Code Reviewer

Reviewed by 2 agents | Quality score: 90% | Review time: 834.1s

---

✅ No Issues Found

All agents reviewed the code and found no issues. LGTM! 🎉

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_{🤖 Generated by AI Code Reviewer | Review ID: review-aa980757}

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