fix: keep remember me sessions alive

[jgordijn]

Summary

• extend PocketBase superuser auth token duration to 30 days on startup
• refresh remembered superuser sessions on app startup and before requests when expiry is near
• add coverage for the superuser auth token duration behavior
• sync the archived remember-me OpenSpec into openspec/specs/remember-me/spec.md
• document the clarified remembered-session behavior, including refresh-before-expiry and clearing invalid remembered sessions

Testing

• go test ./cmd/knowledgehub/...

Notes

• ui typecheck currently reports unrelated pre-existing issues elsewhere in the UI codebase