Choose a Backend

Background: Native [iOS / Android / both] app. Here's what it needs from
a backend: [e.g., user accounts, data sync, file storage, real-time
updates, server-side logic]. Expected scale: [users / data volume].
Team backend experience: [none / some / strong]. Budget sensitivity:
[low / high].

Use case: Help me choose a backend approach and commit, instead of
defaulting to whatever I used last time.

Implementation details: Compare the realistic options:
- Backend-as-a-Service (Firebase, Supabase, Appwrite)
- A custom backend (REST or GraphQL) I host
- A hybrid (BaaS for auth/storage, custom for business logic)
Weigh speed-to-ship, cost at my scale, vendor lock-in, offline support,
and how each is consumed from a native mobile client.

Layer & dependencies: Whatever I choose must sit behind my repository
layer so the rest of the app doesn't depend on the vendor's SDK directly.

Deliverable: A recommendation with reasoning, a comparison table against
my needs, the biggest lock-in risk, an exit/migration sketch if I
outgrow it, and how I'd wrap the SDK so it stays swappable.