Support type field `map` during pass through env variable

## Problem
Confita doesn't support map types when loading from environment variables:

```
type Config struct {
Settings map[string][]int `config:"settings"`
}
```

## Current error
```
Error: failed to parse config: field type 'map' not supported
```

## Proposal
Add support for maps by parsing JSON from env variables:

```bash
export SETTINGS='{"key1":[1,2,3],"key2":[4,5,6]}'
export ACT_RECEIPT_UZ_ALLOWED_INVOICE_PAYMENT_METHODS_BY_SERVICES='{7: [1101], 37: [1101], 671: [1101]}'
```

## Benefits
- More flexible configuration options
- Support for complex nested structures
- Consistency with other config libraries (viper, envconfig)

Start by locating the environment-variable backend and its existing field-type parsing, then trace how unsupported map fields produce the reported error. Add coverage for the shown map[string][]int configuration and verify that valid JSON from an environment variable is decoded into the map without breaking existing types.

Written by the indexing model from the issue text.