Publish structured payloads on contract events

[iexwr]

Context

Events in src/request.rs are emitted as env.events().publish((Symbol, id), ()) — the payload is empty, so the backend indexer cannot read fields without re-deriving state from storage.

Acceptance criteria

• request_created publishes (request_id, payee, asset, amount, memo, expires_at).
• request_paid publishes (request_id, payer).
• request_cancelled publishes (request_id).
• The event payload shapes are documented in README.md (Contract reference table).

Technical notes

• Soroban event topics must be Serde-serializable; use Vec/tuple of Vals via IntoVal, or define #[contracttype] structs for payloads.
• Keep the first topic (the event name Symbol) stable — the backend filters on it.

Testing

• Add a test asserting emitted event topics/body via env.events().all() (testutils).