Add savings goal progress tracking

[SlaVKsVolks]

Summary

• add optional savings goal fields to groups
• add admin set_savings_goal configuration
• track cumulative contributions toward the goal during contributions
• emit a goal-reached event when progress reaches the goal
• add get_progress view returning 0-100 percentage completion
• cover goal progress and reached-state behavior in tests

Closes #45

Verification

• cargo fmt --all
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo build