feat: add member reputation tracking

[Scottcjn]

Summary

• add on-chain MemberReputation storage keyed by member address
• track on_time_contributions each time a member contributes successfully
• increment groups_completed for all members when a group reaches completion
• expose a get_reputation query with zeroed defaults for members without history
• add tests covering default reputation and lifecycle-driven reputation updates

Notes

• defaults is stored in the reputation model and currently remains 0 because the contract does not yet implement a default/missed-payment flow

Testing

• cargo test

Closes #21

[a638011]

Closing: This bounty has no specified dollar amount and does not meet my criteria (USD > $1). Apologies for the noise.

[a638011]

Withdrawing this PR. This bounty has no specified dollar amount and does not meet my criteria (USD > $1). Apologies for the noise.