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/jobs/_market_context.py

@@ -42,6 +42,7 @@ async def latest_quotes_by_group(session) -> dict[str, list[dict]]:
             & (Quote.symbol == sub.c.symbol)
             & (Quote.fetched_at == sub.c.mx),
         )
+        .where(Quote.price.is_not(None))
         .order_by(Quote.group_name, Quote.symbol)
     )
     rows = (await session.execute(stmt)).scalars().all()