EPIC: Finish contributions module

[AgustinBadi]

This Epic tracks remaining work to complete and harden the contribution lifecycle in the new architecture.

The foundational Contributions module CRUD and validation are already implemented in this branch.
This Epic now focuses only on unresolved lifecycle, contract, security, and e2e reliability gaps.

Progress

• Create CRUD for contributions #12 Create CRUD for contributions
• feat(contributions): enforce lifecycle transitions for participant contribution flow #101 Enforce lifecycle transitions for participant contribution flow
• fix(contributions): align storage temporary-state endpoints with actions client contract #102 align storage temporary-state endpoints with actions client contract
• security(contributions): derive contributor identity from JWT in storage multipart operations #103 Derive contributor identity from JWT in storage multipart operations
• #1034 Add e2e coverage for full contribution lifecycle and race scenarios

Goals

• Make contribution lifecycle transitions explicit, safe, and test-backed.
• Remove contract drift between backend APIs and actions client.
• Prevent identity/authorization bypass in contribution upload flows.
• Increase confidence with e2e coverage for lifecycle and concurrency.

Out of Scope

• Circuits module refactors and controller cleanup.
• VM/disk-size/circuit orchestration technical debt not directly required for contribution lifecycle.