fix: use since cursor in github-repo polls to prevent missed commits

After initial backfill, scan_repo was fetching only page 1 (100 most
recent commits) per repo. If more than 100 commits landed between
7-day polls, older ones in that window were permanently missed.

Now stores the newest commit date in poller_state.last_modified and
passes it as &since= on subsequent polls, with full pagination, so
only genuinely new commits are fetched but none are skipped.

On first poll after deploy, last_modified is NULL so no since filter
is applied — triggering a full re-backfill that catches any
previously missed commits.

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

crates/moments-data/src/github_repo.rs

@@ -117,19 +117,23 @@ impl GithubRepoSource {
         Ok(repos)
     }
 
-    /// Fetch commits for a single repo, paginating fully on first run.
+    /// Fetch commits for a single repo, paginating fully on first run
+    /// and using `since` on subsequent runs to catch everything new.
     async fn scan_repo(&self, repo: &Repo) -> Result<usize, SourceError> {
         let state_key = format!("github-repo:{}", repo.full_name);
         let prior = self.state.load(&state_key).await?;
-        let first_run = prior.is_none();
-        let max_pages = if first_run { MAX_BACKFILL_PAGES } else { 1 };
+        let since = prior.as_ref().and_then(|s| s.last_modified);
 
         let mut total = 0usize;
-        for page in 1..=max_pages {
-            let url = format!(
+        let mut newest: Option<DateTime<Utc>> = since;
+        for page in 1..=MAX_BACKFILL_PAGES {
+            let mut url = format!(
                 "https://api.github.com/repos/{}/commits?author={}&per_page={}&page={}",
                 repo.full_name, self.config.user, self.config.per_page, page
             );
+            if let Some(since_dt) = since {
+                url.push_str(&format!("&since={}", since_dt.to_rfc3339()));
+            }
             let req = self.apply_headers(self.client.get(&url));
             let resp = req
                 .send()
@@ -165,6 +169,13 @@ impl GithubRepoSource {
                 .iter()
                 .filter_map(|item| parse_commit(item, repo))
                 .collect();
+            for ev in &events {
+                newest = Some(match newest {
+                    Some(n) if ev.occurred_at > n => ev.occurred_at,
+                    Some(n) => n,
+                    None => ev.occurred_at,
+                });
+            }
             total += self.writer.upsert_events(&events).await?;
 
             if items.len() < self.config.per_page as usize {
@@ -172,7 +183,7 @@ impl GithubRepoSource {
             }
         }
 
-        self.state.touch(&state_key).await?;
+        self.state.save(&state_key, None, newest).await?;
         Ok(total)
     }
 }