feat(3-1): stale state detection and dimmed display

When fetch times out or returns empty after prior data, retain last aircraft
with is_stale=True and render them as black outlines so the display stays
informative rather than blank or crashing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/planemapper/renderer/aircraft.py

@@ -5,7 +5,12 @@ import math
 
 from PIL import Image, ImageDraw, ImageFont
 
-from planemapper.constants import COLOUR_TRAIL, TRAIL_DOT_SIZE_MAX, TRAIL_DOT_SIZE_MIN
+from planemapper.constants import (
+    COLOUR_STALE_OUTLINE,
+    COLOUR_TRAIL,
+    TRAIL_DOT_SIZE_MAX,
+    TRAIL_DOT_SIZE_MIN,
+)
 from planemapper.models import Aircraft
 from planemapper.renderer.colours import altitude_to_colour
 
@@ -78,5 +83,8 @@ def draw_aircraft(
     colour = altitude_to_colour(aircraft.altitude_ft)
     draw = ImageDraw.Draw(image)
     _draw_trail(draw, trail)
-    _draw_arrow(draw, cx, cy, aircraft.heading or 0.0, colour, aircraft.is_mlat)
+    if aircraft.is_stale:
+        _draw_arrow(draw, cx, cy, aircraft.heading or 0.0, COLOUR_STALE_OUTLINE, is_mlat=True)
+    else:
+        _draw_arrow(draw, cx, cy, aircraft.heading or 0.0, colour, aircraft.is_mlat)
     _draw_label(draw, cx, cy, aircraft, colour)