feat(multi-search): add metrics recording to multi-search endpoint (P5.11 §13.11)

Add missing Prometheus metrics to the /multi-search endpoint:
- miroir_multisearch_queries_per_batch: histogram tracking query count per batch
- miroir_multisearch_batches_total: counter for total batches processed
- miroir_multisearch_partial_failures_total: counter for batches with >=1 failed query

The core MultiSearchExecutor and HTTP endpoint were already implemented.
This commit completes the observability requirements from plan §13.11.

All acceptance criteria covered by existing tests:
- 5-query batch: test_five_query_batch_all_complete
- Parallel execution: test_slow_query_doesnt_block_fast_queries
- 100-query batch: test_large_batch_completes
- Partial failure: test_partial_failure_one_error

Closes: miroir-uhj.11

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/miroir-proxy/src/routes/multi_search.rs

@@ -173,6 +173,13 @@ where
         return Err(StatusCode::BAD_REQUEST);
     }
 
+    // Record multi-search metrics (plan §13.11)
+    let query_count = body.queries.len() as u64;
+    state
+        .metrics
+        .observe_multisearch_queries_per_batch(query_count);
+    state.metrics.inc_multisearch_batches_total();
+
     let executor = MultiSearchExecutor::new(state.config.multi_search.clone());
 
     // Get topology and policy once for all queries
@@ -421,6 +428,12 @@ where
             StatusCode::INTERNAL_SERVER_ERROR
         })?;
 
+    // Record partial failures metric (plan §13.11)
+    let has_failures = response.results.iter().any(|r| !r.is_success());
+    if has_failures {
+        state.metrics.inc_multisearch_partial_failures();
+    }
+
     Ok(Json(response))
 }