Add config-driven system.yaml and multi-window indicators

- Create config/system.yaml with full master prompt template
- Create src/config_loader.py to load YAML config and .env
- Update data_engine.build_all_features() to support lists of windows
  (e.g. sma_windows: [3, 20] produces sma_3 and sma_20 columns)
- Rewrite get_candles.py to loop over all instruments × granularities
  from config instead of hardcoding EUR_USD/M5
- Fix supabase_upload.py indentation bug and wire up config for table name

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

src/config_loader.py

@@ -0,0 +1,44 @@
+# src/config_loader.py
+"""
+Loads config/system.yaml and config/.env for the fx-quant project.
+"""
+
+from pathlib import Path
+from dotenv import load_dotenv
+import yaml
+
+
+def get_project_root() -> Path:
+    """Return the fx-quant project root (parent of src/)."""
+    return Path(__file__).resolve().parents[1]
+
+
+def load_config(path=None) -> dict:
+    """
+    Load config/system.yaml and config/.env.
+
+    Parameters
+    ----------
+    path : str or Path, optional
+        Explicit path to a YAML config file.  Defaults to
+        <project_root>/config/system.yaml.
+
+    Returns
+    -------
+    dict
+        Parsed YAML contents.
+    """
+    root = get_project_root()
+
+    # Always load .env as a side effect
+    env_path = root / "config" / ".env"
+    if env_path.exists():
+        load_dotenv(env_path)
+
+    if path is None:
+        path = root / "config" / "system.yaml"
+
+    with open(path, "r") as f:
+        cfg = yaml.safe_load(f)
+
+    return cfg