feat(api-keys): show 'Last used' per API key (closes #492)

[cristim]

Summary

• Switch the "Last used" cell in the API Keys table from formatDateTime (absolute) to formatRelativeTime (e.g. "2h ago") with the full ISO timestamp on a title attribute for hover
• Null last_used_at continues to render "Never" (not a zero time)
• No new migration needed: last_used_at TIMESTAMPTZ NULL already exists in 000001_initial_schema.up.sql, UpdateLastUsed already fires async in ValidateUserAPIKey, and ListUserAPIKeysAPI already propagates the field

Test plan

• Go: TestService_ListUserAPIKeysAPI extended with two new subtests asserting LastUsedAt round-trips correctly (non-nil for used keys, nil for never-used keys)
• TS: two new tests in apikeys.test.ts covering null ("Never") and populated (relative label + ISO title) rendering
• go test ./internal/auth/... ./internal/api/...: 1854 passed
• npx tsc --noEmit: clean
• Frontend jest (apikeys.test.ts): 58 passed

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6c1f63a3-ce79-4bad-b112-378857677429

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and 473680f.

📒 Files selected for processing (3)

• frontend/src/__tests__/apikeys.test.ts
• frontend/src/apikeys.ts
• internal/auth/service_apikeys_api_test.go

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/492-wave19

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