feat(contracts): emit contract events from state-changing entrypoints

[Mona-i]

Description

None of the contracts publish events. State-changing calls — treasury::deposit, governance::create_proposal, voting::cast_vote, dividends::fund/record_share — mutate storage silently. Off-chain consumers (the future @lumio/sdk indexer, the dashboard) have no event stream to follow, which is the idiomatic Soroban way to expose state changes.

Scope

• Publish a contract event from each state-changing entrypoint using env.events().publish(...) with a small, documented topic/data shape (e.g. topic ("deposit", member), data amount).
• Keep events minimal and consistent across contracts.
• Add tests asserting the event is emitted (Soroban testutils env.events().all()).

Out of scope

• An off-chain indexer (separate repo/phase).
• Changing existing return values.

Acceptance criteria

• Each state-changing entrypoint emits a documented event.
• Tests assert emission for at least treasury::deposit and voting::cast_vote.
• Event topics/data are documented (in code doc-comments or the README).
• fmt/build/test green.

Where to start

• contracts/treasury/src/lib.rs:31, contracts/voting/src/lib.rs:32, contracts/governance/src/lib.rs:39, contracts/dividends/src/lib.rs:28.
• Events docs: https://developers.stellar.org/docs/build/smart-contracts/example-contracts/events

Estimated effort: ~4–6h. Good Soroban issue.