feat: document templates with auto-fill from case data (P1)

- Database: kanzlai.document_templates table with RLS policies
- Seed: 4 system templates (Klageerwiderung UPC, Berufungsschrift,
  Mandatsbestätigung, Kostenrechnung)
- Backend: TemplateService (CRUD + render), TemplateHandler with
  endpoints: GET/POST /api/templates, GET/PUT/DELETE /api/templates/{id},
  POST /api/templates/{id}/render?case_id=X
- Template variables: case.*, party.*, tenant.*, user.*, date.*, deadline.*
- Frontend: /vorlagen page with category filters, template detail/editor,
  render flow (select case -> preview -> copy/download), variable toolbar
- Quick action: "Schriftsatz erstellen" button on case detail page
- Also: resolved merge conflicts between audit-trail and role-based branches,
  added missing Notification/AuditLog types to frontend

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"`
+}