refactor: replace config.yml with .env for all configuration

All settings (auth credentials, scanner ranges, status_checker interval)
now live in a single .env file via pydantic-settings. config.yml and
config.yml.example are deleted.

- Settings: add auth_username, auth_password_hash, scanner_ranges,
  status_checker_interval; add load_overrides()/save_overrides() for
  persisting runtime changes to data/scan_config.json
- auth.py: read credentials directly from settings
- scan.py: read ranges/interval from settings; write-back via save_overrides()
- scheduler.py: read interval directly from settings
- main.py: call settings.load_overrides() at startup
- docker-compose.yml: remove config.yml volume mount, add new env vars
- conftest.py: set settings fields directly instead of writing a temp config.yml

backend/app/core/config.py

@@ -1,3 +1,6 @@
+import json
+from pathlib import Path
+
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -6,12 +9,43 @@ class Settings(BaseSettings):
 
     secret_key: str  # Required — set SECRET_KEY in .env
     sqlite_path: str = "./data/homelab.db"
-    config_path: str = "./config.yml"
     cors_origins: list[str] = ["http://localhost:5173", "http://localhost:3000"]
 
     # JWT
     algorithm: str = "HS256"
     access_token_expire_minutes: int = 1440  # 24h
 
+    # Auth — set AUTH_USERNAME and AUTH_PASSWORD_HASH in .env
+    auth_username: str = "admin"
+    auth_password_hash: str = ""
 
-settings = Settings()  # type: ignore[call-arg]  # pydantic-settings loads secret_key from env
+    # Scanner
+    scanner_ranges: list[str] = ["192.168.1.0/24"]
+
+    # Status checker
+    status_checker_interval: int = 60
+
+    def _override_path(self) -> Path:
+        return Path(self.sqlite_path).parent / "scan_config.json"
+
+    def load_overrides(self) -> None:
+        """Load runtime scan config overrides written by the API."""
+        try:
+            data = json.loads(self._override_path().read_text())
+            if "scanner_ranges" in data:
+                self.scanner_ranges = data["scanner_ranges"]
+            if "status_checker_interval" in data:
+                self.status_checker_interval = int(data["status_checker_interval"])
+        except Exception:
+            pass
+
+    def save_overrides(self) -> None:
+        """Persist scan config so it survives container restarts."""
+        self._override_path().parent.mkdir(parents=True, exist_ok=True)
+        self._override_path().write_text(json.dumps({
+            "scanner_ranges": self.scanner_ranges,
+            "status_checker_interval": self.status_checker_interval,
+        }))
+
+
+settings = Settings()  # type: ignore[call-arg]