Replace annular cloud veil with full-frame east sky backdrop

The annular-ring veil approach sampled too small a region (38 px moon
in 3840×2160 gives a tiny annulus) and was invisible in practice.
More fundamentally, using the whole east sky is what "relayed to where
it was taken" actually means.

New approach — _make_east_sky_backdrop():
  Scale the full east camera frame to output size, apply GaussianBlur
  r = output_width // 6 (≈ 320 px at 1920 wide).  The blur erases RTSP
  artefacts, OSD overlays, and the wide-angle look while preserving real
  sky colour, cloud / haze gradients, and the dark ground silhouette.
  The NASA moon disk is pasted sharp on top.  The result reads as east
  photographed the moon through a telephoto with its actual sky that night.

  Clear dark sky → nearly black backdrop (same feel as before).
  Thin cloud cover → soft grey-blue haze behind the sharp moon.
  Heavy overcast → the moon detection would not qualify, so this case
    never reaches rendering.

Config rename: MOON_CLOUD_OVERLAY_* → MOON_EAST_SKY_ENABLED / _BLUR.
moon_phase_monthly.py: CLOUD_OVERLAY_* → EAST_SKY_*.
render_phase_closeup(): cloud_overlay_* params → east_sky_*.

test_moon_composite.py: replace full_moon_closeup.jpg (timestamped
photograph) with a procedural grey disc generated via PIL + numpy so the
test does not look like it is reusing an existing image.

https://claude.ai/code/session_01HkTxpNSTWtViZzxbrbKytR

sky-cam.conf

@@ -524,26 +524,24 @@ MOON_HEIGHT_PCT=0.92               # moon disk fills this fraction of frame heig
 MOON_DIALAMOON_TARGET_PX=2048      # cached PNG longest side; downsampled on save
 MOON_DIALAMOON_TIMEOUT_SEC=30
 
-# Atmospheric cloud veil ──────────────────────────────────────────────────────
-# When east verifies a moon sighting, the surrounding sky region (annulus
-# just outside the moon disk) is sampled and scaled to fill the output frame,
-# then blurred heavily so it reads as atmospheric haze rather than an upscaled
-# photo.  The result is blended over the NASA composite at an opacity
-# proportional to how bright that sky patch was — dark clear sky adds nothing,
-# thin clouds add a gentle veil, bright overcast reaches MOON_CLOUD_OVERLAY_MAX_OPACITY.
+# East sky backdrop ───────────────────────────────────────────────────────────
+# When east verifies a moon sighting, its full camera frame is scaled to
+# output size and blurred heavily (GaussianBlur r ≈ output_width / 6) to
+# produce an atmospheric backdrop behind the NASA moon disk.
 #
-# This is the honest solution to "east doesn't capture high-res clouds": we use
-# east's actual sky as an atmospheric fingerprint rather than pretending to
-# photograph cloud detail that isn't there.
+# The blur removes RTSP artefacts, OSD text, and the wide-angle look while
+# preserving the real sky colour, any cloud or haze gradients, and the dark
+# tree/ground silhouette at the bottom of frame.  The NASA moon is pasted
+# sharp on top — the result looks like east shot it through a telephoto, with
+# its actual sky that night as the background.
 #
-# Opacity scale:
-#   mean sky brightness < 5%   → no veil (clear dark sky)
-#   mean sky brightness   10%  → ~17% veil  (thin haze / airglow)
-#   mean sky brightness ≥ 20%  → MOON_CLOUD_OVERLAY_MAX_OPACITY (40%)
+# This is the honest solution to "east doesn't capture high-res clouds": east's
+# real atmospheric fingerprint (dark and clear, softly hazy, or cloud-diffused)
+# becomes the background without pretending to photograph detail that isn't there.
 #
-MOON_CLOUD_OVERLAY_ENABLED=true
-MOON_CLOUD_OVERLAY_MAX_OPACITY=0.40  # 0.0–1.0; never fully obscures the NASA render
-#MOON_CLOUD_OVERLAY_BLUR=0           # blur radius in px; 0 = auto (output_width / 10)
+# Set false to use a plain black background instead (original behaviour).
+MOON_EAST_SKY_ENABLED=true
+#MOON_EAST_SKY_BLUR=0               # blur radius in px; 0 = auto (output_width / 6)
 
 # ── Mattermost — daily sunrise upload ─────────────────────────────────────────
 # mattermost_url, access_token, channel_id go in .env (see bottom of this file).