Replace combined filament dropdown with separate material + color dropdowns

- app/constants.py: MATERIALS and COLORS standard value lists
- Migration 0008: job_material + job_color columns on print_jobs
- Job creation (both /jobs and /brackets/{id}/jobs) stores these independently
  of the filament inventory
- All job tables updated to display job_material · job_color

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

app/constants.py

@@ -0,0 +1,9 @@
+MATERIALS = ["PLA", "PETG", "ABS", "ASA", "TPU", "Nylon", "PC", "Other"]
+
+COLORS = [
+    "Black", "White", "Grey", "Silver",
+    "Red", "Orange", "Yellow",
+    "Green", "Blue", "Purple", "Pink",
+    "Brown", "Beige", "Natural",
+    "Transparent", "Translucent",
+]