fix: add keeper wallet SOL balance monitoring (C4)

[0x-SquidSol]

Summary

• The keeper wallet pays transaction fees but had zero balance monitoring — when SOL ran out, all transactions silently failed
• Adds startup + periodic (60s) balance checks via connection.getBalance()
• Alerts via Discord at 0.05 SOL (warning) and 0.01 SOL (critical)
• Health endpoint degrades to "down" when balance is critical (can't send txs)
• Exposes wallet section in /health response with address, balance, and status

What's added

Feature	Detail
Startup log	Wallet address + balance logged on boot
Periodic check	Every 60s, piggybacks on existing stale oracle interval
Warning alert	At 0.05 SOL — sendWarningAlert to Discord
Critical alert	At 0.01 SOL — sendCriticalAlert to Discord
Alert rate-limiting	Max one alert per 5 minutes
Health endpoint	wallet.address, wallet.balanceLamports, wallet.balanceSol, wallet.balanceStatus
Health degradation	Balance < 0.01 SOL → status "down" (HTTP 503)
Error handling	Balance check failure logged at debug, never crashes keeper

Test plan

• All 133 existing tests pass
• Zero type errors in index.ts (pre-existing SDK type errors unrelated)
• Balance check wrapped in try/catch — RPC failure is non-fatal
• checkWalletBalance tested implicitly via startup flow

🤖 Generated with Claude Code

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ℹ️ Review info

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Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 44c6ebf3-a5b6-4ba4-8350-e567a85cbfd2

📥 Commits

Reviewing files that changed from the base of the PR and between 63ad1a0 and a5d65ec.

📒 Files selected for processing (1)

• src/index.ts

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