feat: implement in-flight capacity tracking and backpressure for storage provider selection

[alikhere]

Summary

Implements in-flight capacity tracking and backpressure mechanism to prevent overloaded storage nodes from being selected. This ensures that providers with insufficient capacity are automatically excluded during selection, and tracks both used and claimed (in-flight) capacity.

Core Features

• Provider Capacity Tracking: Tracks usedCapacity, claimedCapacity, and maxCapacity per provider
• Capacity-Aware Router: Automatically filters out providers without available capacity
• In-Flight Tracking: Claims capacity during allocation, finalizes on success, releases on failure
• Allocation Tracker: Monitors in-flight allocations with timeout-based cleanup for abandoned uploads

Implementation Details

1. Provider Capacity Storage (ProviderCapacityStorage)

   • In-memory implementation for testing
   • Atomic capacity operations to prevent race conditions
   • Methods: claimCapacity(), releaseClaimed(), finalizeAllocation(), hasAvailableCapacity()

2. Capacity-Aware Router (createCapacityAwareRouter)

   • Wraps existing router to filter providers by capacity
   • Checks used_capacity + claimed_capacity + upload_size <= max_capacity
   • Works for both primary allocation and replication

3. Allocation Flow Integration

   • Allocation: Claims capacity immediately after provider selection
   • Acceptance: Finalizes allocation (claimed → used) on success, releases on failure
   • Failure Handling: Releases claimed capacity on all failure paths

4. Allocation Tracker

   • Tracks in-flight allocations with timestamps
   • Provides cleanup mechanism for abandoned allocations (5-minute default timeout)
   • Prevents memory leaks in allocation accounting

Testing

• ✅ Provider capacity storage operations
• ✅ Prevention of over-allocation
• ✅ Capacity-aware router selection
• ✅ Capacity lifecycle (claim → finalize/release)

Closes #654

[alikhere]

Hi @alanshaw I’ve implemented in-flight capacity tracking and backpressure as discussed in #654.
Please take a look when you have time.