Add a staking pool stats endpoint returning pool balance, total staked, and recent fee inflow per key

[Chucks1093]

Summary

The key detail page needs staking pool data to render the APY estimate. A GET /keys/:keyId/staking endpoint that reads staking pool balance, total staked quantity, and recent protocol fee inflow for the key should be added.

Scope

• Add GET /keys/:keyId/staking endpoint (no auth required)
• Return stakingPoolBalance, totalStaked, recentFeeInflow (sum of fees in the past 30 days), and stakerCount
• Read staking state from on-chain contract storage via the Soroban RPC
• Cache the response in Redis for 60 seconds and invalidate on each stake, unstake, or fee collection event
• Return 404 for unknown key IDs

Acceptance Criteria

• All required fields returned correctly
• recentFeeInflow sums fees for the past 30 days
• Redis cache served within 60-second TTL
• Cache invalidated after stake, unstake, or fee event
• 404 returned for unknown key ID

ETA: 12 hours

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