Endpoint for batch rule subscription

[bracyw]

Changes

Added POST /rules/subscribe endpoint for batch rule subscription. Takes rule_ids and client_id in request body.

Implementation validates all rules exist before subscribing to any (atomic behavior). O(n) time complexity as specified.

Test Cases

• Success case with multiple rules
• Error handling for nonexistent rules
• Empty list edge case
• Concurrent subscription stress test

All tests pass.

Checklist

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• All checks passing
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Closes #497