refactor(storage): extract repeated TTL boilerplate into helpers (#236)

[ZacLou]

Closes #236

What

Extracts the repeated five-line TTL-extension boilerplate into a new storage module, eliminating four copy-paste sites and fixing the missing instance-TTL bump in cancel_circle.

New contracts/sharibo/src/storage.rs

• load_circle(env, id) — load a Circle from persistent storage, panic with CircleNotFound if absent.
• save_circle(env, id, circle) — persist a Circle, extend its persistent TTL, and bump the instance TTL.
• bump_instance(env) — extend instance TTL (used internally by save_circle).
• LEDGER_THRESHOLD and LEDGER_EXTEND_TO constants moved here with their full doc comments.

Changes to lib.rs

• Replaced all env.storage().persistent().get(...).unwrap_or_else(...) calls with load_circle.
• Replaced all set / extend_ttl / extend_ttl triplets with save_circle.
• Bugfix: cancel_circle now also bumps the instance TTL (it was missing the env.storage().instance().extend_ttl(...) call).
• LEDGER_THRESHOLD and LEDGER_EXTEND_TO are no longer duplicated in lib.rs.

Verification

• cargo test should still pass (21 tests).
• cargo build --target wasm32-unknown-unknown --release should succeed.
• No error variants were added, renamed, or removed.
• No discriminants were changed.