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/embedding/pipeline.rs

@@ -103,7 +103,7 @@ async fn embed_page(
     total: usize,
     progress_callback: &Option<Box<dyn Fn(usize, usize)>>,
 ) -> Result<()> {
-    let mut all_chunks: Vec<ChunkWork> = Vec::new();
+    let mut all_chunks: Vec<ChunkWork> = Vec::with_capacity(pending.len() * 3);
     let mut page_normal_docs: usize = 0;
 
     for doc in pending {
@@ -159,7 +159,7 @@ async fn embed_page(
         page_normal_docs += 1;
     }
 
-    let mut cleared_docs: HashSet<i64> = HashSet::new();
+    let mut cleared_docs: HashSet<i64> = HashSet::with_capacity(pending.len());
 
     for batch in all_chunks.chunks(BATCH_SIZE) {
         let texts: Vec<String> = batch.iter().map(|c| c.text.clone()).collect();