fix(recommender): remove Todoist fallback on orchestrator failure; add snooze exclusion

When fetchOrchestratorTip returned null (LiteLLM timeout, bad JSON, etc.)
the recommender silently fell back to randomPolicy, serving a raw Todoist
task with no rationale — explaining both reported symptoms.

- Remove randomPolicy/signalToCandidate; return 204 when orchestrator fails
  so the UI shows "All clear" instead of a confusing Todoist task
- Pass recent_tip through the stack (frontend → POST /recommend →
  fetchOrchestratorTip → ml/serving RecommendRequest → build_orchestrator_messages)
  so after snooze the LLM is instructed not to repeat the snoozed content

Fixes #122

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/prompts.py

@@ -161,16 +161,21 @@ def build_orchestrator_messages(
     hour_of_day: int,
     day_of_week: int,
     science_destiny: int = 50,
+    recent_tip: str | None = None,
 ) -> list[dict]:
     """Build the [system, user] message list for the orchestrator LLM call.
 
     agent_outputs: list of {agent_id, prompt_text} dicts.
     Falls back to raw task summary when agent_outputs is empty.
+    recent_tip: content of a tip the user just snoozed — generate something different.
     """
     style_hint = _science_destiny_instruction(science_destiny)
     system = _SYS_V4_ORCHESTRATOR + (f"\n\n{style_hint}" if style_hint else "")
 
     lines = [f"Current time: {hour_of_day:02d}:00, day_of_week={day_of_week}", ""]
+    if recent_tip:
+        lines.append(f"The user snoozed this tip (do NOT repeat it or anything similar): \"{recent_tip}\"")
+        lines.append("")
     if agent_outputs:
         lines.append("Context from analysis agents:")
         for s in agent_outputs: