Provider abstraction: providers/ package with BaseDataProvider template

One class per provider (TwelveData, Oanda, Tiingo), all subclassing
BaseDataProvider which owns cache, validation/cleaning, freshness +
boundary guarantees, fetch_range pagination, and streaming stubs —
subclasses implement only _fetch_raw. config.json data_providers.active
selects the live feed; data_provider.py / oanda_provider.py stay as
back-compat shims. Module 16 provider-abstraction tests (253 total green).

Claude-Session: https://claude.ai/code/session_01Mc7XKYyPySVXiUpcN82QEK

app_config.py

@@ -282,15 +282,27 @@ def telegram_channel_id(app: Optional[AppConfig] = None) -> str:
     return env if env is not None else app.telegram_channel_id
 
 
-def provider_api_key(app: Optional[AppConfig] = None) -> str:
-    """API key for the ACTIVE provider. The env override is per-provider
-    (TWELVE_DATA_API_KEY applies only when twelve_data is active)."""
+# Per-provider env-var overrides for API keys (env → config.json → "").
+_PROVIDER_ENV_KEYS = {
+    "twelve_data": "TWELVE_DATA_API_KEY",
+    "tiingo": "TIINGO_API_KEY",
+}
+
+
+def provider_api_key(
+    app: Optional[AppConfig] = None, provider: Optional[str] = None
+) -> str:
+    """API key for `provider` (default: the ACTIVE provider). The env
+    override is per-provider (TWELVE_DATA_API_KEY applies only to
+    twelve_data, TIINGO_API_KEY only to tiingo)."""
     app = app or get_app_config()
-    if app.active_provider == "twelve_data":
-        env = os.environ.get("TWELVE_DATA_API_KEY")
+    name = provider or app.active_provider
+    env_key = _PROVIDER_ENV_KEYS.get(name)
+    if env_key:
+        env = os.environ.get(env_key)
         if env:
             return env
-    block = app.provider_settings.get(app.active_provider, {})
+    block = app.provider_settings.get(name, {})
     return str(block.get("api_key", ""))