refactor(savings): return typed errors from tag mutators

[webmirroring]

Summary

• convert add_tags_to_goal and remove_tags_from_goal missing-goal paths from panic! to SavingsGoalError::GoalNotFound
• convert owner-mismatch paths from panic! to SavingsGoalError::Unauthorized
• add error doc comments and update existing tests to assert typed errors

Validation

• cargo fmt -p savings_goals -- --check
• git diff --check

Attempted but blocked locally:

• cargo test -p savings_goals tags_to_goal --locked -- --nocapture
• Blocker: local Cargo is 1.84.0 and cannot parse locked dependency base64ct 1.8.3 because it requires edition2024/Rust 1.85+. This environment has no rustup; the repo toolchain is stable, so CI with newer stable should be able to run it.

Security notes

• Authorization requirement via caller.require_auth() is unchanged.
• Invalid tag content behavior is preserved.
• Only the public ABI error surface changes for goal-not-found and owner-mismatch tag mutations.

Fixes #1015