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/app/routes/conditions.py

@@ -0,0 +1,46 @@
+"""Veil conditions readout — the sky and the field, in one call.
+
+Composes the two real-world signals the app already computes:
+
+  sky          - app.celestial.veil_thinness(): moon phase/illumination and
+                 (when a longitude is offered) proximity to true solar
+                 midnight. Pure math, always present.
+  geomagnetic  - app.geomagnetic.geomagnetic_cache.reading(): NOAA's
+                 planetary Kp from cache. May be None on a cold cache or
+                 when NOAA is unreachable — that is a valid answer, not an
+                 error, and the frontend simply shows fewer lines.
+
+`lon` is accepted as a raw string and parsed by hand so a malformed or
+out-of-range value degrades to the moon-only reading instead of a 422 —
+the conditions strip must never fail because a browser sent junk.
+"""
+
+import math
+
+from fastapi import APIRouter
+
+from app.celestial import veil_thinness
+from app.geomagnetic import geomagnetic_cache
+
+router = APIRouter(prefix="/api", tags=["conditions"])
+
+
+def _parse_longitude(raw: str | None) -> float | None:
+    """A usable longitude in [-180, 180], or None for anything else."""
+    if raw is None:
+        return None
+    try:
+        lon = float(raw)
+    except (TypeError, ValueError):
+        return None
+    if not math.isfinite(lon) or not -180.0 <= lon <= 180.0:
+        return None
+    return lon
+
+
+@router.get("/conditions")
+async def get_conditions(lon: str | None = None):
+    return {
+        "sky": veil_thinness(_parse_longitude(lon)),
+        "geomagnetic": await geomagnetic_cache.reading(),
+    }