fix: include chains in /status when sync_state is empty

[o-az]

Fixes /status returning an empty chains array when the API-visible database has no sync_state rows for configured chains.

Before this change, the endpoint built chains exclusively from sync_state, so it could return ok, version, and rev while omitting all configured chains. That showed up in production even though normal /query requests for both chain IDs still worked.

This updates the handler to fall back to the configured pool map and emit a placeholder status entry for each configured chain when sync_state is empty or unavailable. That keeps the response shape stable and ensures /status reflects configured chains instead of silently dropping them.

Also adds a regression test covering the previously-missed case: empty sync_state, configured chain still present in /status. Existing seeded-path /status integration coverage still passes.

[o-az]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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