perf: Optimize SQL queries and reduce allocations in hot paths

Change detection queries (embedding/change_detector.rs):
- Replace triple-EXISTS subquery pattern with LEFT JOIN + NULL check
- SQLite now scans embedding_metadata once instead of three times
- Semantically identical: returns docs needing embedding when no
  embedding exists, hash changed, or config mismatch

Count queries (cli/commands/count.rs):
- Consolidate 3 separate COUNT queries for issues into single query
  using conditional aggregation (CASE WHEN state = 'x' THEN 1)
- Same optimization for MRs: 5 queries reduced to 1

Search filter queries (search/filters.rs):
- Replace N separate EXISTS clauses for label filtering with single
  IN() clause with COUNT/GROUP BY HAVING pattern
- For multi-label AND queries, this reduces N subqueries to 1

FTS tokenization (search/fts.rs):
- Replace collect-into-Vec-then-join pattern with direct String building
- Pre-allocate capacity hint for result string

Discussion truncation (documents/truncation.rs):
- Calculate total length without allocating concatenated string first
- Only allocate full string when we know it fits within limit

Embedding pipeline (embedding/pipeline.rs):
- Add Vec::with_capacity hints for chunk work and cleared_docs hashset
- Reduces reallocations during embedding batch processing

Backoff calculation (core/backoff.rs):
- Replace unchecked addition with saturating_add to prevent overflow
- Add test case verifying overflow protection

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

src/core/backoff.rs

@@ -8,7 +8,7 @@ pub fn compute_next_attempt_at(now: i64, attempt_count: i64) -> i64 {
     let jitter_factor = rand::thread_rng().gen_range(0.9..=1.1);
     let delay_with_jitter = (capped_delay_ms as f64 * jitter_factor) as i64;
 
-    now + delay_with_jitter
+    now.saturating_add(delay_with_jitter)
 }
 
 #[cfg(test)]
@@ -82,4 +82,11 @@ mod tests {
         let result = compute_next_attempt_at(now, i64::MAX);
         assert!(result > now);
     }
+
+    #[test]
+    fn test_saturating_add_prevents_overflow() {
+        let now = i64::MAX - 10;
+        let result = compute_next_attempt_at(now, 30);
+        assert_eq!(result, i64::MAX);
+    }
 }