feat(admin): GET /admin/users/:id/credentials with on-chain verification (#410)

[ZacLou]

What

Adds GET /api/v1/admin/users/:id/credentials (#410): all credentials for a user, each with its mint transaction verified live against Stellar Horizon, so admins see real on-chain state instead of trusting a stored hash.

Implementation

• CredentialService.getAdminUserCredentials(userId) — joins credentials → courses for titles (same shape as the user-facing list()), then resolves per-row verification:
  • mintTxHash == null → {kind: "none", status: "unknown"}
  • pending_indexer_confirmation placeholder (bad-seq recovery path) → pending without a Horizon lookup, same convention as reward.getTransactions
  • otherwise stellarClient.getHorizonTransaction(hash) → confirmed / pending / failed with ledger + confirmation count
• Admin route GET /:id/credentials on admin-users.routes (authGuard + adminGuard), Fastify swagger schema matching the module's existing routes.
• Cached 30s under a separate credentials:admin-list:<userId> key — deliberately not shared with the user-facing credentials:list: key (different consumer, different TTL).

Notes

• verification.ledger / verification.confirmations come straight from the Horizon verifier already used by rewards transactions, so no new Stellar client code.
• Superseded-by-mistake rows can't appear here — credentials has a unique (userId, courseId) index and mint() is the only writer.

Closes #410

Testing

• New tests/unit/admin/admin-user-credentials.test.ts (4 cases): on-chain resolution paths, the pending-placeholder mapping, cache hit short-circuit, and the 30s TTL. All 4 pass (vitest run).
• Repo-wide tsc --noEmit is unchanged by this PR (main currently carries its own pre-existing errors; this branch adds no new ones — verified by diffing against a clean worktree of 4765abb).