Add stamp pool feature for low-latency stamp provisioning (Issue #63)

[crtahlin]

Summary

• Implements the Stamp Pool feature for low-latency stamp provisioning
• Maintains a reserve of pre-purchased postage stamps that can be released immediately (<5 seconds) instead of waiting for blockchain confirmation time (>1 minute)
• Background task monitors pool levels, syncs from Bee node, and replenishes as needed

Key Components

• StampPoolManager (app/services/stamp_pool.py): Core service managing the pool, background task, and Bee node integration
• Pool API endpoints (app/api/endpoints/pool.py):
  • GET /api/v1/pool/status - Get pool status and reserve levels
  • POST /api/v1/pool/acquire - Acquire stamp from pool instantly
  • GET /api/v1/pool/available - List available stamps
  • POST /api/v1/pool/check - Trigger manual maintenance
• Configuration: 9 new STAMP_POOL_* environment variables
• Documentation: docs/stamp-pool-guide.md

Configuration

STAMP_POOL_ENABLED=false  # Master switch
STAMP_POOL_RESERVE_SMALL=1   # Depth 17 stamps
STAMP_POOL_RESERVE_MEDIUM=1  # Depth 20 stamps  
STAMP_POOL_RESERVE_LARGE=0   # Depth 22 stamps

Test plan

• Unit tests for StampPoolManager (26 tests passing)
• API endpoint tests (status, acquire, available, check)
• Pool disabled state tests (503 responses)
• Low reserve warning logic tests

Closes #63