fix: Project-scoped job claiming, structured rate-limit logging, RRF total_cmp

Targeted fixes across multiple subsystems:

dependent_queue:
- Add project_id parameter to claim_jobs() for project-scoped job claiming,
  preventing cross-project job theft during concurrent multi-project ingestion
- Add project_id parameter to count_pending_jobs() with optional scoping
  (None returns global counts, Some(pid) returns per-project counts)

gitlab/client:
- Downgrade rate-limit log from warn to info (429s are expected operational
  behavior, not warnings) and add structured fields (path, status_code)
  for better log filtering and aggregation

gitlab/transformers/discussion:
- Add tracing::warn on invalid timestamp parse instead of silent fallback
  to epoch 0, making data quality issues visible in logs

ingestion/merge_requests:
- Remove duplicate doc comment on upsert_label_tx

search/rrf:
- Replace partial_cmp().unwrap_or() with total_cmp() for f64 sorting,
  eliminating the NaN edge case entirely (total_cmp treats NaN consistently)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitlab/client.rs

@@ -144,11 +144,12 @@ impl GitLabClient {
 
             if response.status() == StatusCode::TOO_MANY_REQUESTS && attempt < Self::MAX_RETRIES {
                 let retry_after = Self::parse_retry_after(&response);
-                tracing::warn!(
-                    retry_after_secs = retry_after,
+                tracing::info!(
+                    path = %path,
                     attempt,
-                    path,
-                    "Rate limited by GitLab, retrying"
+                    retry_after_secs = retry_after,
+                    status_code = 429u16,
+                    "Rate limited, retrying"
                 );
                 sleep(Duration::from_secs(retry_after)).await;
                 continue;
@@ -565,11 +566,12 @@ impl GitLabClient {
 
             if response.status() == StatusCode::TOO_MANY_REQUESTS && attempt < Self::MAX_RETRIES {
                 let retry_after = Self::parse_retry_after(&response);
-                tracing::warn!(
-                    retry_after_secs = retry_after,
+                tracing::info!(
+                    path = %path,
                     attempt,
-                    path,
-                    "Rate limited by GitLab, retrying"
+                    retry_after_secs = retry_after,
+                    status_code = 429u16,
+                    "Rate limited, retrying"
                 );
                 sleep(Duration::from_secs(retry_after)).await;
                 continue;