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/src/pages/printing/spoolQrCodePrintingDialog.tsx

@@ -157,8 +157,8 @@ const SpoolQRCodePrintingDialog: React.FC<SpoolQRCodePrintingDialog> = ({ spoolI
     `**{filament.vendor.name} - {filament.name}
 #{id} - {filament.material}**
 Spool Weight: {filament.spool_weight} g
-{ET: {filament.settings_extruder_temp} °C}
-{BT: {filament.settings_bed_temp} °C}
+{ET: {filament.settings_extruder_temp_min}-{filament.settings_extruder_temp_max} °C}
+{BT: {filament.settings_bed_temp_min}-{filament.settings_bed_temp_max} °C}
 {Lot Nr: {lot_nr}}
 {{comment}}
 {filament.comment}
@@ -201,6 +201,10 @@ Spool Weight: {filament.spool_weight} g
     { tag: "filament.comment" },
     { tag: "filament.settings_extruder_temp" },
     { tag: "filament.settings_bed_temp" },
+    { tag: "filament.settings_extruder_temp_min" },
+    { tag: "filament.settings_extruder_temp_max" },
+    { tag: "filament.settings_bed_temp_min" },
+    { tag: "filament.settings_bed_temp_max" },
     { tag: "filament.color_hex" },
     { tag: "filament.multi_color_hexes" },
     { tag: "filament.multi_color_direction" },