Add support for overriding configuration via command line arguments

[alexwoo-awso]

Changes

Adds support for overriding .env configuration via command-line arguments in fetch_invoices.py and documents it in README.md.

Key improvements:

• CLI args take precedence over env vars (falls back gracefully if not provided).
• No breaking changes—existing .env-only usage works unchanged.

Added Arguments

python fetch_invoices.py --token YOUR_TOKEN --nip 1234567890 [--format json]

- `--token`: Overrides `KSEF_TOKEN` from `.env`.
- `--nip`: Overrides `CONTEXT_NIP` from `.env`.
- `--format`: Output format (`json` default or `text`).

Code Changes

fetch_invoices.py (13 additions, 5 deletions):

• Updated main(): Load dotenv first, then override with args.token or os.getenv('KSEF_TOKEN').
• Added argparse for --token and --nip with Polish help text.
• Error handling preserved for missing config.

README.md (13 additions, 0 deletions):

• New section: "Nadpisywanie konfiguracji" with usage example and param list.

Usage Examples

With overrides (no .env needed):

python fetch_invoices.py --token twój-token-ksef --nip 1234567890 --format text

With partial overrides:

python fetch_invoices.py --token twój-token-ksef --format json  # Uses .env NIP

Output unchanged: JSON or text summary of fetched invoices.

Testing
Verified: Args override env; fallback works; formats preserved.

No impact on core logic (auth, fetch, decrypt, dedup). Ready to merge.

Feedback welcome!