feat: change volume from string to integer type

Parser:
- Change volume type from Option<String> to Option<i32>
- Parse volume as integer to remove leading zeros
- Keep original title with volume info

Indexer:
- Update SQL queries to insert volume as integer
- Add volume column to INSERT and UPDATE statements

API:
- Change BookItem.volume and BookDetails.volume to Option<i32>
- Add natural sorting for books

Backoffice:
- Update volume type to number
- Update book detail page
- Add CSS styles

crates/parsers/Cargo.toml

@@ -7,4 +7,5 @@ license.workspace = true
 [dependencies]
 anyhow.workspace = true
 lopdf = "0.35"
+regex = "1"
 zip = { version = "2.2", default-features = false, features = ["deflate"] }