Add linecount field to Version database model

This adds the `linecounts` field to both the `Version` struct and `NewVersion` builder. The field stores linecount data as `JSON`, following the established pattern for flexible schema evolution without requiring additional migrations.

The `linecounts` field is `Optional` to handle existing versions that don't have this data, and will be populated for new versions during the publish process. This design ensures backward compatibility while enabling rich source code metrics for future crate versions.

Author: Turbo87

crates/crates_io_database/src/models/version.rs

@@ -36,6 +36,7 @@ pub struct Version {
     pub homepage: Option<String>,
     pub documentation: Option<String>,
     pub repository: Option<String>,
+    pub linecounts: Option<serde_json::Value>,
 }
 
 impl Version {
@@ -103,6 +104,7 @@ pub struct NewVersion<'a> {
     repository: Option<&'a str>,
     categories: Option<&'a [&'a str]>,
     keywords: Option<&'a [&'a str]>,
+    linecounts: Option<serde_json::Value>,
 }
 
 impl NewVersion<'_> {