feat(api): implement /extract and /validate endpoints with error handling

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_api.py

@@ -0,0 +1,95 @@
+import base64
+
+import pytest
+from fastapi.testclient import TestClient
+
+from src.main import app
+
+
+@pytest.fixture
+def client():
+    return TestClient(app)
+
+
+def test_extract_valid_zugferd(client):
+    with open("tests/fixtures/EN16931_Einfach.pdf", "rb") as f:
+        pdf_base64 = base64.b64encode(f.read()).decode()
+
+    response = client.post("/extract", json={"pdf_base64": pdf_base64})
+    assert response.status_code == 200
+    data = response.json()
+    assert data["is_zugferd"] is True
+    assert data["zugferd_profil"] == "EN16931"
+    assert "xml_raw" in data
+    assert "xml_data" in data
+    assert "pdf_text" in data
+    assert "extraction_meta" in data
+
+
+def test_extract_non_zugferd(client):
+    with open("tests/fixtures/EmptyPDFA1.pdf", "rb") as f:
+        pdf_base64 = base64.b64encode(f.read()).decode()
+
+    response = client.post("/extract", json={"pdf_base64": pdf_base64})
+    assert response.status_code == 200
+    data = response.json()
+    assert data["is_zugferd"] is False
+    assert data["zugferd_profil"] is None
+    assert "pdf_text" in data
+    assert "extraction_meta" in data
+
+
+def test_extract_invalid_base64(client):
+    response = client.post("/extract", json={"pdf_base64": "invalid!!!"})
+    assert response.status_code == 400
+    data = response.json()
+    assert data["error"] == "invalid_base64"
+    assert "message" in data
+
+
+def test_extract_non_pdf(client):
+    pdf_base64 = base64.b64encode(b"Hello World").decode()
+    response = client.post("/extract", json={"pdf_base64": pdf_base64})
+    assert response.status_code == 400
+    data = response.json()
+    assert "error" in data
+
+
+def test_validate_pflichtfelder(client):
+    response = client.post(
+        "/validate",
+        json={
+            "xml_data": {
+                "invoice_number": "RE-001",
+                "invoice_date": "2025-02-04",
+                "supplier": {"name": "Test GmbH", "vat_id": "DE123456789"},
+                "buyer": {"name": "Kunde AG"},
+                "totals": {
+                    "net": 100.0,
+                    "gross": 119.0,
+                    "vat_total": 19.0,
+                    "line_total_sum": 100.0,
+                },
+                "line_items": [
+                    {
+                        "position": 1,
+                        "description": "Test",
+                        "quantity": 1.0,
+                        "unit": "Stück",
+                        "unit_price": 100.0,
+                        "line_total": 100.0,
+                    }
+                ],
+            },
+            "checks": ["pflichtfelder"],
+        },
+    )
+    assert response.status_code == 200
+    data = response.json()
+    assert "result" in data
+    assert "is_valid" in data["result"]
+
+
+def test_validate_empty_checks(client):
+    response = client.post("/validate", json={"xml_data": {}, "checks": []})
+    assert response.status_code == 200