fix: rewrite subtask editing to reuse add form instead of inline table row

Edit button now opens the same "Nuova Misurazione" form pre-filled with
existing data, showing "Modifica Misurazione" title and "Aggiorna Misurazione"
button. Also adds marker_number to SubtaskUpdate schema so edits persist.

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

server/schemas/task.py

@@ -20,6 +20,7 @@ class SubtaskCreate(BaseModel):
 
 class SubtaskUpdate(BaseModel):
     """Schema for updating a subtask."""
+    marker_number: Optional[int] = Field(None, ge=1)
     description: Optional[str] = Field(None, min_length=1, max_length=500)
     measurement_type: Optional[str] = Field(None, max_length=100)
     nominal: Optional[float] = None