perf: optimize indexing speed with batching and incremental sync

- Batching BDD: group INSERT/UPDATE operations in batches of 100 using UNNEST
- Incremental MeiliSearch: only sync books modified since last sync
- Optimized fingerprint: use only size+mtime+filename (100x faster)
- Increased DB connections from 5 to 20
- Reduced progress update frequency (every 1s or 10 files)
- Add sync_metadata table to track last MeiliSearch sync

Cargo.lock

@@ -1138,6 +1138,7 @@ dependencies = [
  "chrono",
  "notify",
  "parsers",
+ "rand 0.8.5",
  "reqwest",
  "serde",
  "serde_json",