feat: dodaj prefill email potpisa i body logging

Uvodi signature polje i default predlozak potpisa za nove korisnike, te data migraciju koja popunjava postojece prazne potpise. Email flow sada dosljedno dodaje korisnicki potpis i sprema finalni body poruke u dispatch log radi audita.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/core/users/admin.py

@@ -9,7 +9,7 @@ class CustomUserAdmin(UserAdmin):
     list_display = ('email', 'first_name', 'last_name', 'occupation', 'work_position', 'residence', 'is_serviser', 'is_kupac', 'is_team_member', 'is_active', 'client_profile')
     fieldsets = (
         (None, {'fields': ('email', 'password')}),
-        ('Osobni podaci', {'fields': ('first_name', 'last_name', 'telefon', 'oib', 'occupation', 'work_position', 'residence')}),
+        ('Osobni podaci', {'fields': ('first_name', 'last_name', 'telefon', 'oib', 'occupation', 'work_position', 'residence', 'signature')}),
         ('ERP Status', {'fields': ('is_active', 'is_staff', 'is_superuser', 'is_serviser', 'is_kupac', 'is_team_member', 'is_verified', 'client_profile')}),
         ('Važni datumi', {'fields': ('last_login', 'date_joined')}),
     )
@@ -18,7 +18,7 @@ class CustomUserAdmin(UserAdmin):
     add_fieldsets = (
         (None, {
             'classes': ('wide',),
-            'fields': ('email', 'password', 'is_team_member'),
+            'fields': ('email', 'password', 'first_name', 'last_name', 'telefon', 'occupation', 'signature', 'is_team_member'),
         }),
     )