fix: improve piptar upload - sequential uploads via background task queue (#5994)

* feat: improve piptar upload with sequential background task queue

Replace simultaneous piptar uploads with sequential processing via background job queue while keeping dependency installation parallelized.

Key changes:
- Add PiptarUpload job kind and database migration  
- Queue piptar uploads instead of using tokio::spawn
- Implement job handler for sequential S3 uploads
- Maintain parallel dependency installation as requested

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Ruben Fiszel <rubenfiszel@users.noreply.github.com>

* Remove PiptarUpload job kind migration files

Refactoring piptar uploads to use tokio channel instead of Windmill jobs.
Migration files are no longer needed.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Ruben Fiszel <rubenfiszel@users.noreply.github.com>

* Refactor piptar uploads from Windmill jobs to tokio channel

Replace the complex job queue infrastructure for piptar uploads with a simple 
tokio channel approach as requested. This maintains sequential upload behavior
while keeping dependency installation parallel.

Key changes:
- Add PIPTAR_UPLOAD_CHANNEL global channel for sequential processing
- Replace JobPayload::PiptarUpload with simple channel send
- Remove PiptarUpload from JobKind enum and all job handling code
- Remove job dispatcher case from worker.rs
- Simplify upload logic while maintaining same functionality

Benefits:
- Reduced complexity by removing unnecessary job infrastructure
- Sequential uploads without blocking dependency installation
- Better separation of concerns

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Ruben Fiszel <rubenfiszel@users.noreply.github.com>

* Update worker.rs

* Update python_executor.rs

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: Ruben Fiszel <rubenfiszel@users.noreply.github.com>
Co-authored-by: Ruben Fiszel <ruben@windmill.dev>

backend/windmill-queue/src/jobs.rs

@@ -4028,6 +4028,7 @@ pub async fn push<'c, 'd>(
         ),
     };
 
+
     let final_priority: Option<i16>;
     #[cfg(not(feature = "enterprise"))]
     {