Resolve repo_id to the processing placement in entire api and recap

entire api filled {repo_id} with the first active mirror's id and sent --to cell requests to the home cell. A repo mirrored in several regions has a different id per placement, and only the processing placement's cell holds its data: other cells reject the id or, for the processing id, return an empty result. Recap had the same first-mirror lookup for its ?repo= team scope.

Both now use resolveForgeRepoCellPlacement, the lookup trails, explain and experts already share. A --to cell request whose path names {repo_id} goes to that placement's cell; /me/* paths, {repo_id} in the query string, and an explicit --jurisdiction keep their cell. Lookup failures error in entire api and stay best-effort in recap.

Removes the now-unused first-mirror helpers (firstActiveRepoID, forgeToMirrorProvider, isActiveMirror, listMirrorsForRepo).