Fix missing GST on all AER-sourced usage rates and supply charges

AER's Consumer Data Right unitPrice fields are GST-exclusive, confirmed
by comparing a real, currently-effective EnergyAustralia contract's own
GST-inclusive rates against this plan's cached AER data: usage rates and
the supply charge all matched the cached value multiplied by exactly
1.1 (four independent figures). A real AGL bill confirmed the same
convention separately (excl-GST line items, GST added back as one
combined amount at the end). This understated every one of the ~700+
cached comparison plans' costs by ~10% relative to manually-entered
current plans (which were correctly GST-inclusive, taken from real
bills), biasing every "savings vs current" figure across the board.

Feed-in/solar buy-back rates are the exception -- confirmed GST-free in
that same contract's own terms -- so the multiplier applies only to
usage rates and the supply charge.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/aer/tests/test_aer_mapper.py

@@ -5,7 +5,7 @@ from datetime import datetime
 
 import pytest
 
-from app.costing.aer_mapper import map_aer_plan
+from app.costing.aer_mapper import GST_MULTIPLIER, map_aer_plan
 from app.costing.engine import IntervalReading, evaluate
 from app.costing.tariff_models import find_usage_coverage_gaps
 
@@ -26,7 +26,7 @@ def test_maps_real_agl_time_of_use_plan():
     assert plan.retailer == "AGL"
     assert "Energex" in plan.distributors
     assert len(plan.usage_components) == 3  # peak, off-peak, shoulder
-    assert plan.supply_charge_c_per_day == pytest.approx(1.536 * 100)
+    assert plan.supply_charge_c_per_day == pytest.approx(1.536 * 100 * GST_MULTIPLIER)
 
     labels = {c.label for c in plan.usage_components}
     assert {"Time of Use Peak", "Time of Use Off Peak", "Time of Use Shoulder"} <= labels

app/costing/aer_mapper.py

@@ -62,10 +62,27 @@ def _time_windows(entries: list[dict]) -> list[TimeWindow]:
     ]
 
 
+# AER's Consumer Data Right "Get Generic Plan Detail" unitPrice fields are
+# GST-EXCLUSIVE -- confirmed by comparing a real, currently-effective
+# EnergyAustralia contract's own quoted GST-inclusive rates against this
+# plan's cached AER data: every usage rate and the supply charge matched
+# the cached value multiplied by exactly 1.1, precisely (four independent
+# figures, not a coincidence), and a real AGL bill separately confirmed
+# the same convention (its line items are excl-GST, with GST added back as
+# one combined amount at the end). Feed-in/solar buy-back rates are the
+# exception -- confirmed GST-free in that same EnergyAustralia contract's
+# own terms ("The rate at which we purchase your electricity does not
+# include GST") -- so this multiplier applies only to usage rates and the
+# supply charge, never to feed_in_tariffs/tiered_feed_in.
+GST_MULTIPLIER = 1.1
+
+
 def _rate_cents(rates: list[dict]) -> tuple[float, bool]:
-    """Returns (cents_per_kwh, is_tiered). Only the first tier is used for
-    tiered/inclining-block rate structures -- callers should mark the plan
-    not-modelable when is_tiered is True, since later tiers are ignored."""
+    """Returns (cents_per_kwh, is_tiered), GST-exclusive as given by AER --
+    callers apply GST_MULTIPLIER themselves where it applies (usage rates,
+    not feed-in). Only the first tier is used for tiered/inclining-block
+    rate structures -- callers should mark the plan not-modelable when
+    is_tiered is True, since later tiers are ignored."""
     return float(rates[0]["unitPrice"]) * 100, len(rates) > 1
 
 
@@ -84,13 +101,17 @@ def map_aer_plan(list_entry: dict, detail: dict) -> TariffPlan:
     for period in ec.get("tariffPeriod", []):
         season_months = _season_months(period.get("startDate", "01-01"), period.get("endDate", "12-31"))
         rate_type = period.get("rateBlockUType")
-        supply_charges_c.append(float(period.get("dailySupplyCharge", 0) or 0) * 100)
+        supply_charges_c.append(float(period.get("dailySupplyCharge", 0) or 0) * 100 * GST_MULTIPLIER)
 
         if rate_type == "singleRate":
             block = period["singleRate"]
             rate_c, tiered = _rate_cents(block["rates"])
             usage_components.append(
-                UsageComponent(label=block.get("displayName", "Usage"), rate_c_per_kwh=rate_c, season_months=season_months)
+                UsageComponent(
+                    label=block.get("displayName", "Usage"),
+                    rate_c_per_kwh=rate_c * GST_MULTIPLIER,
+                    season_months=season_months,
+                )
             )
             if tiered:
                 modelable = False
@@ -101,7 +122,7 @@ def map_aer_plan(list_entry: dict, detail: dict) -> TariffPlan:
                 usage_components.append(
                     UsageComponent(
                         label=tou_block.get("displayName", tou_block.get("type", "TOU")),
-                        rate_c_per_kwh=rate_c,
+                        rate_c_per_kwh=rate_c * GST_MULTIPLIER,
                         time_windows=_time_windows(tou_block.get("timeOfUse", [])),
                         season_months=season_months,
                     )