Fix estimation: replace 20% infill with 50% material factor, 32 g/h speed

The 20% infill assumption ignored walls and solid layers which dominate
most prints. Validated against Bambu Lab Studio for a GF lid:
  model volume 26.09 cm³ × 0.50 × 1.24 g/cm³ = 16.2 g  (actual: 16.24 g)
  16.2 g / 32 g/h × 60 = 30 min                          (actual: 30.5 min)

New env vars (read at request time, no rebuild needed):
  MATERIAL_FACTOR=0.50        fraction of model volume that becomes filament
  PRINT_SPEED_G_PER_HOUR=32   matches Bambu Lab at normal quality

Card header now shows active assumptions including speed.

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

.env.install

@@ -8,3 +8,5 @@ TZ=Europe/Berlin
 FILAMENTDB_URL=https://web.filamentdb.orb.local
 VAT_RATE=19
 FILAMENT_PRICE_PER_KG=15
+MATERIAL_FACTOR=0.50
+PRINT_SPEED_G_PER_HOUR=32