🛡️ Sentinel: [HIGH] Fix Missing Rate Limiting on Invite Redemption

[arumes31]

🚨 Severity: HIGH
💡 Vulnerability: The Redeem endpoint in internal/handlers/invite.go was missing rate limiting based on IP address.
🎯 Impact: Attackers could automate mass-creation of accounts and enumerate invitation tokens by continuously hitting the endpoint.
🔧 Fix: Added a rate limit check using core.CheckRateLimit with the reg_ip:<IP> key to mitigate brute-force attempts and registration spam. Also updated the corresponding test suite to ensure rate-limiting behaviors are properly verified.
✅ Verification: Successfully verified by running GOTOOLCHAIN=go1.26.4 go test ./... with a new rate-limit test case explicitly passing.

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PR created automatically by Jules for task 13803970017010300009 started by @arumes31

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