feat: sync inventory contract with registry to prevent double-allocation

[KingFRANKHOOD]

Pr Summary

InventoryContract could allocate/reserve the same blood unit independently of the HealthChain registry, creating a race condition where two hospitals get the same unit.

• Closes [Enhancement] InventoryContract and main BloodUnitRegistry manage overlapping unit state with no synchronisation #933
• Closes [backend] FIREBASE_SERVICE_ACCOUNT_JSON placeholder crashes PushProvider with ERROR-level log on every startup #997
• Closes [backend] ApprovalsService and CustodyService modified in refactor but not audited for DI errors #1054
• Closes [backend] No request validation pipe configured globally — DTOs accept arbitrary payloads without sanitisation #1055

What Changed

• Added a cross-contract sync layer. The InventoryContract now checks registry availability before reserving and notifies the registry on reserve/release. The registry's write endpoints are guarded by env.current_contract_address().