refactor: rename apps/ to clients/ + update architecture docs

Rename apps/ → clients/ for clarity:
- app/ (singular) = Python backend (FastAPI, server-rendered web UI)
- clients/ (plural) = standalone client applications (API consumers)

The web storefront/store/admin stays in app/ because it's server-
rendered Jinja2, not a standalone frontend. clients/ is for native
apps that connect to the API externally.

Updated:
- docs/architecture/overview.md — added clients/ to project structure
- clients/terminal-android/SETUP.md — updated path references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/architecture/overview.md

@@ -322,6 +322,11 @@ project/
 │   ├── unit/
 │   └── integration/
 │
+├── clients/            # Standalone client applications (API consumers)
+│   └── terminal-android/  # Native Android POS terminal (Kotlin/Compose)
+│
+├── scripts/            # Seed scripts, deploy scripts
+│
 └── docs/               # Documentation
     ├── architecture/   # System architecture
     ├── frontend/       # Frontend guides