feat: configuration loader with env var overrides

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_config.py

@@ -0,0 +1,35 @@
+import os
+import textwrap
+import pytest
+from fablednetmon.config import load_config
+
+def test_load_minimal_config(tmp_path):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text(textwrap.dedent("""\
+        database:
+          url: postgresql+asyncpg://user:pass@localhost/fablednetmon
+        secret_key: test-secret
+    """))
+    cfg = load_config(cfg_file)
+    assert cfg["database"]["url"] == "postgresql+asyncpg://user:pass@localhost/fablednetmon"
+    assert cfg["secret_key"] == "test-secret"
+
+def test_env_var_overrides_scalar(tmp_path, monkeypatch):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text("database:\n  url: original\nsecret_key: s\n")
+    monkeypatch.setenv("FABLEDNETMON_DATABASE__URL", "overridden")
+    cfg = load_config(cfg_file)
+    assert cfg["database"]["url"] == "overridden"
+
+def test_env_var_does_not_override_list(tmp_path, monkeypatch):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text("smtp:\n  recipients:\n    - a@b.com\nsecret_key: s\ndatabase:\n  url: x\n")
+    monkeypatch.setenv("FABLEDNETMON_SMTP__RECIPIENTS", "bad")
+    cfg = load_config(cfg_file)
+    assert cfg["smtp"]["recipients"] == ["a@b.com"]
+
+def test_missing_required_key_raises(tmp_path):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text("{}")
+    with pytest.raises(ValueError, match="secret_key"):
+        load_config(cfg_file)