chore: drop unreachable branches outside the signal core (no behaviour change)

- providers: _to_oanda_instrument / _to_tiingo_ticker / _to_td_symbol no longer
  guess a wire symbol for pairs that are not registered — the registries are
  the only source, so an unknown symbol raises the provider's DataIntegrityError
  instead of silently fetching something else. Every scanned symbol is validated
  against PAIR_INFO at import, so nothing reachable changes.
- main / telegram_bot _fmt_price: the sub-$10 five-decimal branch could never
  fire for gold.
- news_engine._parse_pair: BASE/QUOTE only; the Yahoo "=X" fallback was
  unreachable (its test goes with it).
- dashboard_publisher.event_to_dict: drop minutes_until (frozen at publish
  time, misleading) and is_high_impact — dashboard.js never read either.
- static/dashboard.html: unused id="cal-table".

Tried and reverted: trimming the LogRow fields nothing in analyze_signal_log
reads — the tests read them to verify the loader round-trips those DB columns,
so they stay.

Suite: 460 passed, 0 failed.

Claude-Session: https://claude.ai/code/session_0156YhogLytzj9P2iH1skGbL

providers/oanda.py

@@ -93,9 +93,11 @@ class OandaDataProvider(BaseDataProvider):
         return self._fetch_oanda_candles(symbol, timeframe, outputsize)
 
     def _to_oanda_instrument(self, symbol: str) -> str:
-        if symbol in self.OANDA_INSTRUMENT:
+        try:
             return self.OANDA_INSTRUMENT[symbol]
-        return symbol.replace("/", "_")
+        except KeyError:
+            raise OandaDataError(
+                f"Unsupported symbol {symbol!r} for Oanda") from None
 
     def _fetch_oanda_candles(
         self,