Add CLAUDE.md with architecture and dev commands

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Common Commands
+
+```bash
+# Activate the venv (required before all commands below)
+source .venv/bin/activate
+
+# Run all acceptance tests
+pytest tests/ -v
+
+# Run a single test
+pytest tests/test_acceptance.py::test_assign_battery -v
+
+# Seed the database (safe to skip if batteries.db already exists)
+python seed.py
+
+# Start the dev server
+flask run
+
+# Start production server (same as systemd service uses)
+waitress-serve --host=127.0.0.1 --port=5000 app:app
+
+# Install/reinstall systemd user service
+bash sbin/install-service.sh
+
+# Service management
+systemctl --user start|stop|restart|status battery-tracker
+journalctl --user -u battery-tracker -f
+```
+
+## Architecture
+
+### Request lifecycle
+`app.py` contains `create_app(config_object)` which builds the Flask app, creates the SQLAlchemy engine from `config.SQLALCHEMY_DATABASE_URI`, calls `Base.metadata.create_all(engine)`, and wires a `scoped_session` as the `db` local. All routes close over this `db` via `@app.teardown_appcontext`. There is no Flask-SQLAlchemy — the ORM session is raw SQLAlchemy so that `models.py` can be imported by `migrate_to_mariadb.py` without an app context.
+
+### Database config
+`config.py` reads `DATABASE_URL` from the environment, falling back to `sqlite:///batteries.db`. Swapping to MariaDB is entirely a matter of setting that env var — no code changes needed. All column types are restricted to `Integer`, `String`, `Text`, `Boolean`, `DateTime` for MariaDB compatibility. Status is stored as `String(20)` (not `Enum`) to avoid DDL differences between SQLite and MariaDB.
+
+### Models
+- `Battery`: label (unique), brand, status (`available`/`installed`/`retired`), device_id (FK nullable, `ondelete=SET NULL`), notes
+- `Device`: name (unique), battery_slots, notes
+- Helper methods on both models (`is_available()`, `installed_count()`, `has_mixed_brands()`) are used directly in templates and route logic.
+
+### Business rules (enforced in routes, not DB constraints)
+- Assigning a retired battery → hard block with flash error
+- Assigning to a full device → hard block with flash error
+- Mixed brands on same device → flash warning, assignment still proceeds
+- Deleting a battery → requires GET confirmation page first, then POST
+
+### Testing
+Tests use a temporary file-based SQLite DB (via `tempfile.mkstemp`) created fresh per test — avoids SQLite in-memory per-connection isolation issues. The `seeded_client` fixture in `conftest.py` pre-populates via HTTP POST calls (not direct DB access), so tests exercise the full stack. Battery IDs in tests are positional (id=1 is always the first battery POSTed by the fixture).
+
+### MariaDB migration
+`migrate_to_mariadb.py` opens two SQLAlchemy sessions simultaneously (SQLite source, MariaDB destination), migrates Devices first (FK dependency), then Batteries with explicit `id=` values to preserve FK links, then resets `AUTO_INCREMENT` via `text("ALTER TABLE ...")`. Takes `MARIADB_URL` from env or CLI arg. See `MIGRATION.md` for the full procedure.