Explain this codebase to me like I have to maintain it

Explain this codebase so I can maintain it confidently.

File tree: <paste the output of your tree or ls -R, trimmed>
Key files (paste the main entry points, routing, data layer): <paste>
What the app does, as far as I know: <one paragraph>
What I want to do next: <the change I'm about to make>

Produce:
1. The shape: what kind of app this is, the framework, how it's structured, in 5 sentences.
2. A map: each top-level folder and what belongs there, one line each.
3. How data flows: from user action to storage and back, as a numbered path with file names.
4. The three files that matter most, and why.
5. Conventions I should follow: naming, where new things go, how errors and state are handled (based on what you see, not general advice).
6. Smells: anything duplicated, unused, or risky that I should know about. Quote file names.
7. For the change I'm about to make: which files I'll touch, what could break, and where to add a test.

Base everything on what I pasted. Where you can't tell, say "can't tell from these files" rather than assuming.