added Validation rules

backend/src/middleware/Validation/Shiftplan.md

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+## Shift Plan Management
+
+### \[CREATE\] shift plan
+* All operations require authentication
+* 401 responses trigger automatic logout
+* Scheduled shifts array is guaranteed to exist (empty array if none)
+
+### \[CREATE\] shift plan from preset
+* presetName must match existing TEMPLATE_PRESETS
+* Requires name, startDate, and endDate
+* isTemplate is optional (defaults to false)
+
+### \[UPDATE\] shift plan
+* Requires valid shift plan ID
+* Partial updates allowed
+* Authentication required
+
+### \[ACTION: delete shift plan\]
+* Requires authentication
+* 401 responses trigger automatic logout
+
+### \[ACTION: regenerate scheduled shifts\]
+* Requires valid plan ID
+* Authentication required
+* Fails silently if regeneration fails (logs error but continues)
+
+### \[ACTION: clear assignments\]
+* Requires valid plan ID
+* Authentication required
+* Clears all employee assignments from scheduled shifts
+
+## ShiftPlan
+
+### \[CREATE\] ShiftPlan from template
+* `planName` must not be empty
+* `startDate` must be set
+* `endDate` must be set
+* `endDate` must be after `startDate`
+* `selectedPreset` must be chosen (template must be selected)
+* Only available template presets can be used
+
+### \[ACTION: publish\] ShiftPlan
+* Plan must be in 'draft' status
+* All active employees must have set their availabilities for the plan
+* Only users with roles \['admin', 'maintenance'\] can publish
+* Assignment algorithm must not have critical violations (ERROR or ❌ KRITISCH)
+* employee && employee.contract_type === small => mind. 1 mal availability === 1 || availability === 2
+* employee && employee.contract_type === large => mind. 3 mal availability === 1 || availability === 2
+
+### \[ACTION: recreate assignments\]
+* Plan must be in 'published' status
+* Only users with roles \['admin', 'maintenance'\] can recreate
+* User confirmation required before clearing all assignments
+
+### \[ACTION: delete\] ShiftPlan
+* Only users with roles \['admin', 'maintenance'\] can delete
+* User confirmation required before deletion
+
+### \[ACTION: edit\] ShiftPlan
+* Only users with roles \['admin', 'maintenance'\] can edit
+* Can only edit plans in 'draft' status
+
+### \[UPDATE\] ShiftPlan shifts
+* `timeSlotId` must be selected from available time slots
+* `requiredEmployees` must be at least 1
+* `dayOfWeek` must be between 1-7