feat: append-only audit trail for all mutations (P0)

- Database: kanzlai.audit_log table with RLS, append-only policies
  (no UPDATE/DELETE), indexes for entity, user, and time queries
- Backend: AuditService.Log() with context-based tenant/user/IP/UA
  extraction, wired into all 7 services (case, deadline, appointment,
  document, note, party, tenant)
- API: GET /api/audit-log with entity_type, entity_id, user_id,
  from/to date, and pagination filters
- Frontend: Protokoll tab on case detail page with chronological
  audit entries, diff preview, and pagination

Required by § 50 BRAO and DSGVO Art. 5(2).

backend/internal/services/deadline_service.go

@@ -1,6 +1,7 @@
 package services
 
 import (
+	"context"
 	"database/sql"
 	"fmt"
 	"time"
@@ -13,12 +14,13 @@ import (
 
 // DeadlineService handles CRUD operations for case deadlines
 type DeadlineService struct {
-	db *sqlx.DB
+	db    *sqlx.DB
+	audit *AuditService
 }
 
 // NewDeadlineService creates a new deadline service
-func NewDeadlineService(db *sqlx.DB) *DeadlineService {
-	return &DeadlineService{db: db}
+func NewDeadlineService(db *sqlx.DB, audit *AuditService) *DeadlineService {
+	return &DeadlineService{db: db, audit: audit}
 }
 
 // ListAll returns all deadlines for a tenant, ordered by due_date
@@ -87,7 +89,7 @@ type CreateDeadlineInput struct {
 }
 
 // Create inserts a new deadline
-func (s *DeadlineService) Create(tenantID uuid.UUID, input CreateDeadlineInput) (*models.Deadline, error) {
+func (s *DeadlineService) Create(ctx context.Context, tenantID uuid.UUID, input CreateDeadlineInput) (*models.Deadline, error) {
 	id := uuid.New()
 	source := input.Source
 	if source == "" {
@@ -108,6 +110,7 @@ func (s *DeadlineService) Create(tenantID uuid.UUID, input CreateDeadlineInput)
 	if err != nil {
 		return nil, fmt.Errorf("creating deadline: %w", err)
 	}
+	s.audit.Log(ctx, "create", "deadline", &id, nil, d)
 	return &d, nil
 }
 
@@ -123,7 +126,7 @@ type UpdateDeadlineInput struct {
 }
 
 // Update modifies an existing deadline
-func (s *DeadlineService) Update(tenantID, deadlineID uuid.UUID, input UpdateDeadlineInput) (*models.Deadline, error) {
+func (s *DeadlineService) Update(ctx context.Context, tenantID, deadlineID uuid.UUID, input UpdateDeadlineInput) (*models.Deadline, error) {
 	// First check it exists and belongs to tenant
 	existing, err := s.GetByID(tenantID, deadlineID)
 	if err != nil {
@@ -154,11 +157,12 @@ func (s *DeadlineService) Update(tenantID, deadlineID uuid.UUID, input UpdateDea
 	if err != nil {
 		return nil, fmt.Errorf("updating deadline: %w", err)
 	}
+	s.audit.Log(ctx, "update", "deadline", &deadlineID, existing, d)
 	return &d, nil
 }
 
 // Complete marks a deadline as completed
-func (s *DeadlineService) Complete(tenantID, deadlineID uuid.UUID) (*models.Deadline, error) {
+func (s *DeadlineService) Complete(ctx context.Context, tenantID, deadlineID uuid.UUID) (*models.Deadline, error) {
 	query := `UPDATE deadlines SET
 	            status = 'completed',
 	            completed_at = $1,
@@ -176,11 +180,12 @@ func (s *DeadlineService) Complete(tenantID, deadlineID uuid.UUID) (*models.Dead
 		}
 		return nil, fmt.Errorf("completing deadline: %w", err)
 	}
+	s.audit.Log(ctx, "update", "deadline", &deadlineID, map[string]string{"status": "pending"}, map[string]string{"status": "completed"})
 	return &d, nil
 }
 
 // Delete removes a deadline
-func (s *DeadlineService) Delete(tenantID, deadlineID uuid.UUID) error {
+func (s *DeadlineService) Delete(ctx context.Context, tenantID, deadlineID uuid.UUID) error {
 	query := `DELETE FROM deadlines WHERE id = $1 AND tenant_id = $2`
 	result, err := s.db.Exec(query, deadlineID, tenantID)
 	if err != nil {
@@ -193,5 +198,6 @@ func (s *DeadlineService) Delete(tenantID, deadlineID uuid.UUID) error {
 	if rows == 0 {
 		return fmt.Errorf("deadline not found")
 	}
+	s.audit.Log(ctx, "delete", "deadline", &deadlineID, nil, nil)
 	return nil
 }