feat(github): per-repo commit enumeration for full history backfill

Adds a new github-repo EventSource that enumerates all repos via
/user/repos and walks each repo's /commits?author= endpoint, which
has no 1000-result cap unlike the Search API. Events use the same
github-commit:{sha} ID scheme as github_search for dedup. Per-repo
poller state enables full backfill on first run, page-1-only on
subsequent polls. Weekly poll interval by default.

Closes #1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/moments-data/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod bugzilla;
 pub mod gitea;
 pub mod github;
+pub mod github_repo;
 pub mod github_search;
 pub mod hg;