feat: recalibrate P.77/P.75 thresholds; capacity guard on alerts

The first ntfy cycle announced "Warning level at P.77" at 3.02 m. That
gauge's 2.85 m threshold sat below its own dry-season baseline (2.6-2.7 m
at 8-14 % channel capacity): P.77 had been "above warning" for 761 of the
last 2 146 hours, at 22 % capacity. Across 2018-2024, 75-85 % capacity
reads 3.35-4.57 m and 95-105 % reads 4.27-5.08 m; set 4.30 / 4.90. P.75
moved 2.75/3.50 -> 3.20/3.65 on the same evidence (2024: 3.45 / 3.72).
The predictor already handles changed thresholds (regression-derived
probabilities until the Oct 1 retrain).

Second line of defence in notify.py: a clear->alert transition is only
announced when RID's discharge_percent for the reading is >= 60 %, so a
re-rated or datum-shifted gauge cannot page subscribers again. P.1 is
exempt (its stages come from the inundation map, not capacity); readings
without a capacity figure fall back to level only; the all-clear edge is
never blocked. 3 tests.

tests/test_notify.py

@@ -182,6 +182,73 @@ def test_stale_feed_and_recovery(pub):
     assert len(pub.sent) == 2 and "recovered" in pub.sent[1].title
 
 
+def test_capacity_guard_blocks_stale_threshold(pub):
+    """P.77 2026-09: 3.02 m >= 2.85 m 'warning' at 22 % capacity -> not a flood."""
+    state = notify.InMemoryState()
+    r = {
+        "station_code": "P.77",
+        "water_level": 4.40,
+        "timestamp": "2026-09-24T12:00:00",
+        "discharge_percent": 10.3,
+    }
+    notify.evaluate([r], [], state, pub, now=NOW)
+    assert pub.sent == [] and state.get("level:P.77") is None
+    # same level with capacity agreeing -> alert
+    r["discharge_percent"] = 82.0
+    notify.evaluate([r], [], state, pub, now=NOW)
+    assert topics(pub) == ["ping-p77-warning", "ping-warning"]
+
+
+def test_capacity_guard_exempts_p1_and_missing_pct(pub):
+    state = notify.InMemoryState()
+    notify.evaluate(
+        [
+            {
+                "station_code": "P.1",
+                "water_level": 3.75,
+                "timestamp": "2026-09-24T12:00:00",
+                "discharge_percent": 40.0,
+            }
+        ],
+        [],
+        state,
+        pub,
+        now=NOW,
+    )
+    assert topics(pub) == ["ping-p1-warning", "ping-warning"]
+    pub.sent.clear()
+    notify.evaluate(
+        [
+            {
+                "station_code": "P.103",
+                "water_level": 6.0,
+                "timestamp": "2026-09-24T12:00:00",
+            }
+        ],
+        [],
+        state,
+        pub,
+        now=NOW,
+    )
+    assert topics(pub) == ["ping-p103-warning", "ping-warning"]
+
+
+def test_capacity_guard_does_not_block_clearing(pub):
+    """Guard applies only to the clear->alert edge; the all-clear always goes out."""
+    state = notify.InMemoryState()
+    r = {
+        "station_code": "P.67",
+        "water_level": 2.6,
+        "timestamp": "2026-09-24T12:00:00",
+        "discharge_percent": 90.0,
+    }
+    notify.evaluate([r], [], state, pub, now=NOW)
+    assert len(pub.sent) == 2
+    r.update(water_level=2.2, discharge_percent=30.0)
+    notify.evaluate([r], [], state, pub, now=NOW)
+    assert "back to normal" in pub.sent[2].title
+
+
 def test_state_survives_restart_via_sql(tmp_path, pub):
     from sqlalchemy import create_engine