feat(reports): reference-baseline mode (typed limits / prior averages)

Baseline can now come from fixed values typed per location, not just captured
data — for a spec limit ("L10 = 85") or a prior report's averages when the raw
data isn't available.

- SoundReportConfig.baseline_mode ("captured" | "reference").
- report_pipeline: _location_reference_baseline() reads per-location values from
  location_metadata; build_*_night_report honor baseline_mode (reference cells
  use the typed value; unset metrics compare against nothing).
- reports router: GET/PUT /reports/baseline (mode on config + per-location values
  in location_metadata); config carries baseline_mode; manual view/run fall back
  to the saved config's baseline when no explicit dates are given.
- orchestrator + scheduler tick thread baseline_mode through.

Verified end-to-end: PUT/GET /baseline, reference deltas (L10 66.6 vs 85 -> -18.4),
unset metrics compare against nothing, captured-mode regression intact.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/services/report_orchestrator.py

@@ -72,6 +72,7 @@ def run_nightly_report(
     *,
     metric_keys: Optional[list[str]] = None,
     windows: Optional[list[Window]] = None,
+    baseline_mode: str = "captured",
     baseline_start: Optional[date] = None,
     baseline_end: Optional[date] = None,
     recipients: Optional[list[str]] = None,
@@ -86,6 +87,7 @@ def run_nightly_report(
     report = build_project_night_report(
         db, project_id, night_date,
         metric_keys=metric_keys, windows=windows,
+        baseline_mode=baseline_mode,
         baseline_start=baseline_start, baseline_end=baseline_end,
     )