Add referral tracking for group joins

[SlaVKsVolks]

Summary

• add optional referred_by support to join_group
• track successful referral counts per member in persistent storage
• expose get_referral_count(member) and emit a referral event for off-chain rewards
• update join tests for the new optional referral argument and cover referred joins

Closes #67

Verification

• cargo fmt --all
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo build
• git diff --check