HTTP + database example

Run the same validated Axum API against three database backends.

The database_api example selects SQLite, PostgreSQL or MySQL from DATABASE_URL and uses the same model for validated HTTP input, JSON responses, table creation and CRUD:

cargo run -p rydantic --example database_api --features axum,sqlite
# Connect the same API to a server database:
DATABASE_URL="$POSTGRES_URL" cargo run -p rydantic --example database_api --features axum,postgres
DATABASE_URL="$MYSQL_URL" cargo run -p rydantic --example database_api --features axum,mysql
# POST /users, GET /users, GET/PUT/DELETE /users/{id}
curl -X POST http://127.0.0.1:3000/users \
  -H 'Content-Type: application/json' \
  -d '{"name":" Alice ","age":25}'

# Run the example without starting a persistent server:
cargo run -p rydantic --example database_api --features axum,sqlite -- --smoke

The sqlite_api example provides a smaller SQLite-only server. The multi-database example defaults to an in-memory database and only accepts loopback bind addresses. Set DATABASE_URL='sqlite://app.db?mode=rwc' for a local database file and LISTEN_ADDR for another loopback address or port. With both integrations enabled, SqlError implements Axum responses: validation errors return 422, missing rows return 404, database constraint violations return 409, and other driver errors return a generic 500 response.