feat: Add temperature ranges for extruder and bed temps (#5)

Store extruder and bed temperatures as min/max ranges instead of
single values for more accurate filament specifications.

Backend changes:
- Add 4 new columns: settings_extruder_temp_min/max, settings_bed_temp_min/max
- Keep old columns for backward compatibility (marked deprecated)
- Alembic migration copies existing values to both min and max
- Update Pydantic models and API endpoints

Frontend changes:
- Filament model updated with new fields
- Create form uses min/max input pairs with Space.Compact
- Import from external DB populates both min and max with same value
- Label template updated to show ranges: "ET: 190-220 °C"
- Template help includes new tags

Labels now display temperature ranges like "190-220°C" which is more
useful for printing than a single recommended value.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/public/locales/en/common.json

@@ -236,6 +236,8 @@
             "comment": "Comment",
             "settings_extruder_temp": "Extruder Temp",
             "settings_bed_temp": "Bed Temp",
+            "temp_min": "Min",
+            "temp_max": "Max",
             "color_hex": "Color",
             "single_color": "Single",
             "multi_color": "Multi",