Understand an unfamiliar codebase fast

I've just been dropped into this codebase and need to ship a change to {{area}} this week.

Given the files below, produce an orientation doc:
1. What this system does, in two sentences, for someone who has never seen it.
2. The request lifecycle: trace one typical operation end to end, naming each file and function it passes through.
3. The data model as it's actually used, not as it's declared — including the fields that matter and the ones that appear dead.
4. Where the important decisions live: config, feature flags, auth, and the places behaviour changes by environment.
5. The conventions I must follow so my PR doesn't look foreign — naming, error handling, logging, test layout — with an example of each from the code.
6. The three parts I should be scared of, and why.
7. The smallest safe first change to make in {{area}}, and what to read before it.

Mark anything you're unsure about rather than guessing.

FILES:
{{paste key files or directory tree}}