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>

tests/test_openrouter_prompt.py

@@ -9,7 +9,7 @@ pytest.importorskip("pydantic_settings")
 
 from datetime import datetime, timezone
 
-from app.services.openrouter import SYSTEM_PROMPT, build_user_prompt
+from app.services.llm_prompts import SYSTEM_PROMPT, build_user_prompt
 
 
 def test_system_prompt_has_voice_anchors():
@@ -35,7 +35,7 @@ def test_pro_tone_falls_back_to_intermediate():
     """PRO was removed in PROMPT_VERSION 6 (audience pivot to young
     investors). Legacy callers that still pass PRO should get the
     INTERMEDIATE prompt rather than a KeyError."""
-    from app.services.openrouter import build_system_prompt
+    from app.services.llm_prompts import build_system_prompt
     pro = build_system_prompt("PRO", "SPECULATIVE")
     inter = build_system_prompt("INTERMEDIATE", "SPECULATIVE")
     assert pro == inter