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/api/routes/auth.py

@@ -1,6 +1,5 @@
 import hmac
 
-import yaml
 from fastapi import APIRouter, HTTPException, status
 from pydantic import BaseModel
 
@@ -20,20 +19,12 @@ class TokenResponse(BaseModel):
     token_type: str = "bearer"
 
 
-def _load_credentials() -> tuple[str, str]:
-    with open(settings.config_path) as f:
-        cfg = yaml.safe_load(f)
-    auth = cfg.get("auth", {})
-    return auth.get("username", "admin"), auth.get("password_hash", "")
-
-
 @router.post("/login", response_model=TokenResponse)
 async def login(body: LoginRequest) -> TokenResponse:
-    username, password_hash = _load_credentials()
     # Always run both checks to prevent timing-based username enumeration.
     # hmac.compare_digest is constant-time; verify_password (bcrypt) always runs.
-    username_ok = hmac.compare_digest(body.username, username)
-    password_ok = verify_password(body.password, password_hash)
+    username_ok = hmac.compare_digest(body.username, settings.auth_username)
+    password_ok = verify_password(body.password, settings.auth_password_hash)
     if not username_ok or not password_ok:
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
     token = create_access_token(body.username)

backend/app/api/routes/scan.py

@@ -1,7 +1,6 @@
 import logging
 from typing import Any
 
-import yaml
 from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException
 from pydantic import BaseModel
 from sqlalchemy import select
@@ -20,19 +19,11 @@ class ScanConfig(BaseModel):
     ranges: list[str]
     interval_seconds: int
 
+
 logger = logging.getLogger(__name__)
 router = APIRouter()
 
 
-def _load_ranges() -> list[str]:
-    try:
-        with open(settings.config_path) as f:
-            cfg: dict[str, Any] = yaml.safe_load(f) or {}
-        return list(cfg.get("scanner", {}).get("ranges", []))
-    except Exception:
-        return []
-
-
 async def _background_scan(run_id: str, ranges: list[str]) -> None:
     async with AsyncSessionLocal() as db:
         await run_scan(ranges, db, run_id)
@@ -44,7 +35,7 @@ async def trigger_scan(
     db: AsyncSession = Depends(get_db),
     _: str = Depends(get_current_user),
 ) -> ScanRun:
-    ranges = _load_ranges()
+    ranges = settings.scanner_ranges
     run = ScanRun(status="running", ranges=ranges)
     db.add(run)
     await db.commit()
@@ -112,25 +103,18 @@ async def list_runs(db: AsyncSession = Depends(get_db), _: str = Depends(get_cur
 
 @router.get("/config", response_model=ScanConfig)
 async def get_scan_config(_: str = Depends(get_current_user)) -> ScanConfig:
-    try:
-        with open(settings.config_path) as f:
-            cfg = yaml.safe_load(f)
-        ranges = cfg.get("scanner", {}).get("ranges", [])
-        interval = int(cfg.get("status_checker", {}).get("interval_seconds", 60))
-        return ScanConfig(ranges=ranges, interval_seconds=interval)
-    except Exception as exc:
-        raise HTTPException(status_code=500, detail=str(exc)) from exc
+    return ScanConfig(
+        ranges=settings.scanner_ranges,
+        interval_seconds=settings.status_checker_interval,
+    )
 
 
 @router.post("/config", response_model=ScanConfig)
 async def update_scan_config(payload: ScanConfig, _: str = Depends(get_current_user)) -> ScanConfig:
     try:
-        with open(settings.config_path) as f:
-            cfg = yaml.safe_load(f) or {}
-        cfg.setdefault("scanner", {})["ranges"] = payload.ranges
-        cfg.setdefault("status_checker", {})["interval_seconds"] = payload.interval_seconds
-        with open(settings.config_path, "w") as f:
-            yaml.dump(cfg, f, default_flow_style=False, allow_unicode=True)
+        settings.scanner_ranges = payload.ranges
+        settings.status_checker_interval = payload.interval_seconds
+        settings.save_overrides()
         return payload
     except Exception as exc:
         raise HTTPException(status_code=500, detail=str(exc)) from exc