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

news_engine.py

@@ -457,11 +457,10 @@ class NewsEngine:
 
     @staticmethod
     def _parse_pair(pair_symbol: str) -> Tuple[str, str]:
-        if "/" in pair_symbol:
-            parts = pair_symbol.split("/")
-            return parts[0], parts[1]
-        clean = pair_symbol.replace("=X", "")
-        return clean[:3], clean[3:]
+        base, sep, quote = pair_symbol.partition("/")
+        if not sep:
+            raise ValueError(f"pair symbol must be BASE/QUOTE, got {pair_symbol!r}")
+        return base, quote
 
     def get_event_risk_level(self, pair_symbol: str, context: FundamentalContext) -> Tuple[str, str]:
         """