feat: document templates with auto-fill (P1)

backend/internal/models/document_template.go

@@ -0,0 +1,21 @@
+package models
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type DocumentTemplate struct {
+	ID          uuid.UUID       `db:"id" json:"id"`
+	TenantID    *uuid.UUID      `db:"tenant_id" json:"tenant_id,omitempty"`
+	Name        string          `db:"name" json:"name"`
+	Description *string         `db:"description" json:"description,omitempty"`
+	Category    string          `db:"category" json:"category"`
+	Content     string          `db:"content" json:"content"`
+	Variables   json.RawMessage `db:"variables" json:"variables"`
+	IsSystem    bool            `db:"is_system" json:"is_system"`
+	CreatedAt   time.Time       `db:"created_at" json:"created_at"`
+	UpdatedAt   time.Time       `db:"updated_at" json:"updated_at"`
+}