feat: switch config to JSONC + split secrets.json

Load business config as JSONC (comments + trailing commas) via an
in-repo stripper then stdlib json — no new deps. Default path prefers
config.jsonc, falls back to legacy config.json. Auth write-back targets
only sibling secrets.json so JSONC comments are never wiped; secrets
overlay merges on load. Examples, compose, Docker, docs, favsort and
tools all share the same loader. Phase A only — no DB settings migration.

tools/seed_preview_db.py

@@ -8,7 +8,7 @@
 用法::
 
     uv run python tools/seed_preview_db.py --days 7 --seed 20261031
-    uv run python app.py serve --preview --config data/preview/config.json
+    uv run python app.py serve --preview --config data/preview/config.jsonc
 """
 
 from __future__ import annotations
@@ -31,7 +31,9 @@ from reaction_board.timeutil import SHANGHAI_TZ, shanghai_datetime  # noqa: E402
 
 DEFAULT_SOURCE = ROOT / "data" / "app.db"
 DEFAULT_TARGET = ROOT / "data" / "preview" / "app.db"
-DEFAULT_CONFIG_SOURCE = ROOT / "config.json"
+from reaction_board.config import default_config_path  # noqa: E402
+
+DEFAULT_CONFIG_SOURCE = default_config_path(ROOT)
 
 BASE_DAILY_RATES: dict[str, float] = {
     "view": 0.0040,
@@ -187,8 +189,9 @@ def _read_source_config(path: Path | None) -> dict[str, Any] | None:
     if path is None or not path.exists():
         return None
     try:
-        data = json.loads(path.read_text(encoding="utf-8"))
-    except (OSError, json.JSONDecodeError):
+        from reaction_board import jsonc
+        data = jsonc.load(path)
+    except (OSError, ValueError):
         return None
     return data if isinstance(data, dict) else None
 
@@ -617,7 +620,7 @@ def seed_preview(
     config_target = (
         Path(config_out).resolve()
         if config_out is not None
-        else target_path.parent / "config.json"
+        else target_path.parent / "config.jsonc"
     )
     config_target.parent.mkdir(parents=True, exist_ok=True)