lore-proto, lore-server: add RepositoryCount RPC

[lorelab-graham]

Summary

Adds RepositoryService.RepositoryCount(RepositoryCountRequest) -> RepositoryCountResponse for callers that want a repository count without paying RepositoryList's per-entry cost.
RepositoryList streams a full Repository proto per entry, which is orders of magnitude more expensive than the count itself.

The new RPC mirrors RepositoryList's filter surface (optional string creator).

• Unfiltered path returns candidate_ids.len() directly and skips every per-repo metadata read, deserialization, and proto encoding that RepositoryList performs
• Filtered path still walks metadata (the creator filter is applied after load, same as RepositoryList) and uses the same JoinSet fan-out to keep tail latency down
• list_candidate_ids promoted to pub(super) so both handlers share the auth-service-vs-list_local split
• Not added to ForwardedRepositoryService; cross-server count aggregation is a separate concern

Test Plan

• Unit tests in repository_count.rs: empty store, N repos with no filter, N repos with creator filter matching M, filter matching none
• Proto shape test in v1_repository.rs destructures the new request and response
• cargo test --workspace passes (no regressions)
• Exercised against live servers. Counts match RepositoryList