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/search/rrf.rs

@@ -60,11 +60,7 @@ pub fn rank_rrf(vector_results: &[(i64, f64)], fts_results: &[(i64, f64)]) -> Ve
         .collect();
 
     // Sort descending by rrf_score
-    results.sort_by(|a, b| {
-        b.rrf_score
-            .partial_cmp(&a.rrf_score)
-            .unwrap_or(std::cmp::Ordering::Equal)
-    });
+    results.sort_by(|a, b| b.rrf_score.total_cmp(&a.rrf_score));
 
     // Normalize: best = 1.0
     if let Some(max_score) = results.first().map(|r| r.rrf_score).filter(|&s| s > 0.0) {