backend: dedupe shared logic (indicator_summary_job, CHAT_REFERENCE_LINE, call_openrouter alias)

- indicator_summary_job.py imported its own copies of _month_spend and
  _latest_quotes_by_group; _market_context.py already exposes these.
  Switched to the canonical imports. Also fixed _market_context's
  latest_quotes_by_group to actually filter null prices (it claimed to
  in its docstring but lacked the WHERE clause).
- api.py duplicated REFERENCE_LINE as CHAT_REFERENCE_LINE — same string,
  two sources of truth. Now imports REFERENCE_LINE.
- Chat endpoint used the deprecated `call_openrouter` alias and passed
  an explicit `model=` that bypassed the provider chain. Switched to
  `call_llm` with default model selection, then removed the alias.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/services/openrouter.py

@@ -775,9 +775,6 @@ async def call_llm(
     raise last_exc
 
 
-# Back-compat alias for any straggling import sites.
-call_openrouter = call_llm
-
 
 def month_window() -> tuple[datetime, datetime]:
     """[start, now] in UTC for the current calendar month."""