Default to rule-based healing (free) instead of Claude ($3/run)

Claude healing is expensive for weekly runs. Rule-based fallback does
the same validation (range checks, gap filling) without API costs.
Claude healing remains available when explicitly enabled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/api.py

@@ -817,7 +817,7 @@ async def _run_pipeline_async():
             None,
             lambda: run_pipeline_sync(
                 days_back=90,
-                use_claude_healer=bool(os.environ.get("ANTHROPIC_API_KEY")),
+                use_claude_healer=False,  # Rule-based by default; Claude healing costs ~$3/run
                 use_claude_explainer=bool(os.environ.get("ANTHROPIC_API_KEY")),
                 delivery_channel="console",
                 db=db if db.is_postgres else None,