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.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0156YhogLytzj9P2iH1skGbL

providers/twelve_data.py

@@ -58,10 +58,10 @@ class TwelveDataProvider(BaseDataProvider):
 
     @staticmethod
     def _to_td_symbol(symbol: str) -> str:
-        if "/" in symbol:
-            return symbol
-        clean = symbol.replace("=X", "")
-        return f"{clean[:3]}/{clean[3:]}"
+        if "/" not in symbol:
+            raise DataIntegrityError(
+                f"Unsupported symbol {symbol!r} for Twelve Data")
+        return symbol
 
     # ── Raw fetch ──────────────────────────────────────────────────