openrouter: split into llm_prompts (prompt engineering) + transport

openrouter.py was 790 lines mixing two orthogonal concerns:
- Prompt engineering (build_system_prompt, build_summary_*,
  build_chat_*, build_daily_digest_*, etc.) — ~400 lines, changes
  weekly as PROMPT_VERSION bumps
- LLM transport (call_llm, _provider_chain, _call_provider, retry
  + fallback machinery) — ~250 lines, rarely changes

Extracted the prompt-engineering surface to app/services/llm_prompts.py.
Transport stays in openrouter.py (consistent with the filename — the
OpenRouter URL is the transport's anchor).

All import sites (jobs, routers, services, tests) split their
multi-import lines into two: prompt-things from llm_prompts, transport
from openrouter. PROMPT_VERSION constant, _TONE_ALIASES, _resolve_tone,
and SYSTEM_PROMPT moved with the prompt functions.

No behaviour change — pure relocation. Function signatures, body, and
naming all preserved.

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

app/jobs/indicator_summary_job.py

@@ -22,13 +22,15 @@ from app.models import (
 )
 from app.services.cadence import DEFAULT_POLICY
 from app.services.i18n import ACTIVE_LANGUAGES
-from app.services.openrouter import (
+from app.services.llm_prompts import (
     PROMPT_VERSION,
-    active_model,
     build_aggregate_summary_system_prompt,
     build_aggregate_summary_user_prompt,
     build_summary_system_prompt,
     build_summary_user_prompt,
+)
+from app.services.openrouter import (
+    active_model,
     call_llm,
     llm_configured,
     month_start,