Document the event decoding functions

[Glittersup]

Summary

The decoder turns raw contract events into typed objects and is where a change to an event shape would first bite. Its functions are undocumented, so the expected input shape is only discoverable by reading the body.

Acceptance criteria

• Each exported decoding function has a doc comment describing its input and output.
• The comment states what happens when an event does not match a known shape.
• The documented shapes match the contract's events.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-backend.git
cd tricklepay-backend
git remote add upstream https://github.com/TricklePay/tricklepay-backend.git

Create a branch for this issue:

git checkout -b docs/issue-126

Suggested commit message:

docs: document the event decoding functions

Run npm run typecheck, npm test, and npm run build before opening a pull request and linking this issue.