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/models.py

@@ -235,7 +235,10 @@ class SoundReportConfig(Base):
     enabled = Column(Boolean, default=False, nullable=False)        # run the daily report?
     report_time = Column(String, default="08:00", nullable=False)   # local HH:MM to run/send
     metric_keys = Column(String, default="lmax,l01,l10,l90", nullable=False)  # csv of metric keys
-    baseline_start = Column(Date, nullable=True)                    # baseline-week range
+    # Baseline source: "captured" = compute from recorded nights in the date range below;
+    # "reference" = use fixed values typed per location (old-report averages or a spec limit).
+    baseline_mode = Column(String, default="captured", nullable=False)
+    baseline_start = Column(Date, nullable=True)                    # captured-mode range
     baseline_end = Column(Date, nullable=True)
     recipients = Column(Text, nullable=True)                        # csv; falls back to REPORT_SMTP_RECIPIENTS env
     last_run_date = Column(Date, nullable=True)                     # evening-date of the last reported night (dedup)