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

apps/indexer/Cargo.toml

@@ -10,6 +10,7 @@ axum.workspace = true
 chrono.workspace = true
 notify = "6.1"
 parsers = { path = "../../crates/parsers" }
+rand.workspace = true
 reqwest.workspace = true
 serde.workspace = true
 serde_json.workspace = true