Make manage.py use the service's database

manage.py only found the database via BACKUP_DASH_DB, which is set in
the systemd unit and not in an admin's shell, so it silently created a
new backup-dashboard.db in the current directory. API keys created
there were never seen by the service.

- Settings now come from /etc/backup-dashboard/backup-dashboard.env,
  which both the systemd unit (EnvironmentFile=) and manage.py read;
  real environment variables still take precedence
- manage.py prints the database it uses and refuses to create a new
  one unless --create is passed; --db overrides the path

Co-Authored-By: Claude Opus 5.5 <[email protected]>

backend/app/settings.py

@@ -5,6 +5,9 @@ from pathlib import Path
 # The modal shows this many days of run history; retention can never be shorter.
 HISTORY_DAYS = 30
 
+# Shared by the systemd service (EnvironmentFile=) and manage.py, so both always use the same DB.
+DEFAULT_ENV_FILE = "/etc/backup-dashboard/backup-dashboard.env"
+
 
 @dataclass(frozen=True)
 class Settings:
@@ -14,9 +17,27 @@ class Settings:
     serve_static: bool = False
 
 
+def read_env_file(path: Path) -> dict[str, str]:
+    """Parse simple KEY=VALUE lines (the subset systemd's EnvironmentFile accepts)."""
+    values = {}
+    try:
+        lines = path.read_text().splitlines()
+    except FileNotFoundError:
+        return values
+    for line in lines:
+        line = line.strip()
+        if not line or line.startswith("#") or "=" not in line:
+            continue
+        key, _, value = line.partition("=")
+        values[key.strip()] = value.strip().strip("\"'")
+    return values
+
+
 def load_settings() -> Settings:
+    # Real environment variables win over the env file; the env file wins over dev defaults.
+    env = read_env_file(Path(os.environ.get("BACKUP_DASH_ENV_FILE", DEFAULT_ENV_FILE))) | os.environ
     return Settings(
-        db_path=Path(os.environ.get("BACKUP_DASH_DB", "backup-dashboard.db")),
-        retention_days=max(HISTORY_DAYS, int(os.environ.get("BACKUP_DASH_RETENTION_DAYS", "90"))),
-        serve_static=os.environ.get("BACKUP_DASH_SERVE_STATIC") == "1",
+        db_path=Path(env.get("BACKUP_DASH_DB", "backup-dashboard.db")),
+        retention_days=max(HISTORY_DAYS, int(env.get("BACKUP_DASH_RETENTION_DAYS", "90"))),
+        serve_static=env.get("BACKUP_DASH_SERVE_STATIC") == "1",
     )