Auto-update config.json with new scalar fields from config.example.json on startup

On each startup, if config.json exists alongside config.example.json, any scalar
fields (strings, ints, booleans) present in the example but absent in the running
config are added automatically and the file is rewritten atomically. Existing values
and dict/list sections are never touched. Added fields are logged at INFO level.

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

app/config.py

@@ -1,11 +1,15 @@
 from __future__ import annotations
 
 import json
+import logging
 import os
+import tempfile
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any
 
+log = logging.getLogger(__name__)
+
 try:
     from dotenv import load_dotenv
 except ModuleNotFoundError:  # pragma: no cover - optional dependency fallback
@@ -88,6 +92,35 @@ def _read_json_file(path: Path) -> dict[str, Any]:
     return json.loads(path.read_text(encoding="utf-8"))
 
 
+def _write_json_file(path: Path, data: dict[str, Any]) -> None:
+    serialized = json.dumps(data, ensure_ascii=False, indent=2)
+    with tempfile.NamedTemporaryFile(
+        "w",
+        encoding="utf-8",
+        delete=False,
+        dir=path.parent,
+        prefix=path.name,
+        suffix=".tmp",
+    ) as handle:
+        handle.write(serialized)
+        temp_path = Path(handle.name)
+    temp_path.replace(path)
+
+
+def _merge_example_defaults(target: dict[str, Any], source: dict[str, Any], path: str = "") -> list[str]:
+    """Add scalar keys present in source but absent in target; recurse into existing dicts."""
+    added: list[str] = []
+    for key, value in source.items():
+        full_path = f"{path}.{key}" if path else key
+        if key not in target:
+            if not isinstance(value, (dict, list)):
+                target[key] = value
+                added.append(full_path)
+        elif isinstance(target[key], dict) and isinstance(value, dict):
+            added.extend(_merge_example_defaults(target[key], value, full_path))
+    return added
+
+
 def _parse_int(value: Any, *, field_name: str) -> int:
     try:
         return int(value)
@@ -350,6 +383,16 @@ def load_app_config(root: Path) -> AppConfig:
     raw: dict[str, Any] = {}
     if candidate.exists():
         raw = _read_json_file(candidate)
+        example_path = root / "config.example.json"
+        if example_path.exists():
+            try:
+                example_raw = _read_json_file(example_path)
+                added = _merge_example_defaults(raw, example_raw)
+                if added:
+                    _write_json_file(candidate, raw)
+                    log.info("Auto-updated %s with new fields: %s", candidate.name, ", ".join(added))
+            except Exception as exc:
+                log.warning("Could not auto-update config from example: %s", exc)
 
     env_api_id = os.environ.get("TG_API_ID")
     env_api_hash = os.environ.get("TG_API_HASH")