chore(pool): move MIN_INITIAL_DEPOSIT to constants.rs (#592)

[ZacLou]

Closes #592

What

Moves the MIN_INITIAL_DEPOSIT constant from lib.rs to constants.rs, where other shared pool constants already live.

Changes

• constants.rs: added pub const MIN_INITIAL_DEPOSIT: u128 = 10_000_000; with its existing doc comment.
• lib.rs: removed the MIN_INITIAL_DEPOSIT definition (it remains available through pub use constants::*;).

Notes

• No behavioural change — the constant is re-exported from constants.rs exactly as before.
• cargo check -p pool should pass.

[mergekeeper]

Needs review

Linked to #592, but the diff does not match the issue scope.

The pull request deletes almost the entire contents of contracts/pool/src/lib.rs, which goes far beyond moving the MIN_INITIAL_DEPOSIT constant.

Reviewed commit: 444152498e7b215e2aa17e93bc49e4bb5b07f922.

[ZacLou]

Closing — rebase error deleted almost the entire lib.rs. Opening a clean replacement PR.