feat: legibility — conditions, hints, kinder errors (Workstream B)

GET /api/conditions surfaces what the backend already computes: moon
phase, veil thinness, geomagnetic state — junk lon degrades to moon-only
instead of a 422, missing NOAA data means fewer lines, never an error.
VeilConditions renders it in the séance side column, polling every 10 min.

ModeHint: one in-fiction line per mode after 15s of an unused sensor,
dismissed forever via localStorage. Error copy in evp/radio now
detect-and-redirects (mic denied -> 'the board needs no ear'; no WebUSB
-> try EVP) instead of dead-ending.

No geolocation prompt from the conditions strip — asking for location
from a passive readout would be hostile; ?lon= stays supported for
callers that have it.

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

# Conflicts:
#	frontend/src/pages/SeancePage.tsx

backend/tests/test_conditions.py

@@ -0,0 +1,73 @@
+"""GET /api/conditions — sky always present, geomagnetic nullable, lon lenient.
+
+The geomagnetic cache is stubbed in every test so nothing here ever
+touches NOAA; a cold real cache would otherwise fire a live HTTP request.
+"""
+
+import pytest
+
+import app.routes.conditions as conditions_module
+
+
+class _StubCache:
+    def __init__(self, reading):
+        self._reading = reading
+
+    async def reading(self):
+        return self._reading
+
+
+@pytest.fixture
+def geo_null(monkeypatch):
+    monkeypatch.setattr(conditions_module, "geomagnetic_cache", _StubCache(None))
+
+
+@pytest.fixture
+def geo_storm(monkeypatch):
+    monkeypatch.setattr(
+        conditions_module,
+        "geomagnetic_cache",
+        _StubCache({"kp": 6.33, "label": "moderate geomagnetic storm",
+                    "disturbance": 6.33 / 9.0, "storm": True}),
+    )
+
+
+@pytest.mark.asyncio
+async def test_sky_always_present_geomagnetic_null_tolerated(client, geo_null):
+    resp = await client.get("/api/conditions")
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["geomagnetic"] is None
+    sky = body["sky"]
+    assert sky["moon_name"]
+    assert 0.0 <= sky["moon_illumination"] <= 1.0
+    assert 0.0 <= sky["thinness"] <= 1.0
+    # No longitude offered — witching proximity is unknowable.
+    assert sky["witching_proximity"] is None
+
+
+@pytest.mark.asyncio
+async def test_valid_lon_yields_witching_proximity(client, geo_null):
+    resp = await client.get("/api/conditions", params={"lon": "-3.7"})
+    assert resp.status_code == 200
+    witching = resp.json()["sky"]["witching_proximity"]
+    assert witching is not None
+    assert 0.0 <= witching <= 1.0
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("bad", ["ghost", "", "nan", "inf", "999", "-181"])
+async def test_invalid_lon_ignored_not_422(client, geo_null, bad):
+    resp = await client.get("/api/conditions", params={"lon": bad})
+    assert resp.status_code == 200
+    assert resp.json()["sky"]["witching_proximity"] is None
+
+
+@pytest.mark.asyncio
+async def test_geomagnetic_reading_passed_through(client, geo_storm):
+    resp = await client.get("/api/conditions")
+    assert resp.status_code == 200
+    geo = resp.json()["geomagnetic"]
+    assert geo["kp"] == pytest.approx(6.33)
+    assert geo["storm"] is True
+    assert geo["label"] == "moderate geomagnetic storm"