feat(embed): concurrent batching, UTF-8 safe chunking, right-sized chunks

Three fixes to the embedding pipeline:

1. Concurrent HTTP batching: fire EMBED_CONCURRENCY (2) Ollama requests
   in parallel via join_all, then write results serially to SQLite.
   ~2x throughput improvement on GPU-bound workloads.

2. UTF-8 boundary safety: all computed byte offsets in split_into_chunks
   (paragraph/sentence/word break finders + overlap advance) now use
   floor_char_boundary() to prevent panics on multi-byte characters
   like smart quotes and non-breaking spaces.

3. CHUNK_MAX_BYTES reduced from 6000 to 1500 to fit nomic-embed-text's
   actual 2048-token context window, eliminating context-length retry
   storms that were causing 10x slowdowns.

Also threads ShutdownSignal through embed pipeline for graceful Ctrl+C.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/commands/sync.rs

@@ -239,7 +239,7 @@ pub async fn run_sync(
                     embed_bar_clone.set_position(processed as u64);
                 }
             });
-            match run_embed(config, options.full, false, Some(embed_cb)).await {
+            match run_embed(config, options.full, false, Some(embed_cb), signal).await {
                 Ok(embed_result) => {
                     result.documents_embedded = embed_result.embedded;
                     embed_bar.finish_and_clear();