FLO-4 fix rebalancer to use positionID

[holyfuchs]

Closes: #213

Summary

Refactors the paid rebalancer to use position-based identifiers and documents the permissionless design.

Changes

• Refactor: Use positionID (and id param labels) instead of uuid across the paid rebalancer and supervisor API.
• One per position: storeRebalancer panics with "rebalancer already exists" if one is already stored for that position; covered by tests.
• Docs: Docstrings updated for permissionless design and security.

Security & constraints

• Permissionless: Anyone with a position rebalance cap can call createPaidRebalancer.
• Spam: Mitigated by existing economic cost of positions (e.g. minimumBalance), not by this contract.

[jordanschalm]

Looks good - thank you for also keeping documentation up to date 🙇