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

config.json.example

@@ -9,6 +9,9 @@
     "active": "twelve_data",
     "twelve_data": {
       "api_key": "REPLACE_WITH_YOUR_TWELVE_DATA_API_KEY"
+    },
+    "tiingo": {
+      "api_key": "REPLACE_WITH_YOUR_TIINGO_API_KEY_OR_EMPTY"
     }
   },
   "scanning": {