feat: add prescription refills column to planner table and email (#207)

- Add 6th column 'Prescription refills' to frontend Planner table
- Add matching column to backend planner email (HTML + plaintext)
- Show remaining refills for meds with prescription tracking, '–' otherwise
- Add backend translations for new column header (EN + DE)
- Add frontend i18n keys for prescription refills column
- Update planner tests with medications table schema

Closes #203

frontend/src/i18n/de.json

@@ -207,6 +207,7 @@
       "medication": "Medikament",
       "usage": "Verbrauch",
       "blistersNeeded": "Blister benötigt",
+      "prescriptionRefills": "Rezept-Nachfüllungen",
       "blisters": "Blister",
       "available": "Verfügbar"
     }

frontend/src/i18n/en.json

@@ -207,6 +207,7 @@
       "medication": "Medication",
       "usage": "Usage",
       "blistersNeeded": "Blisters needed",
+      "prescriptionRefills": "Prescription refills",
       "blisters": "Blisters",
       "available": "Available"
     }