switch to poi-tl for template rendering, add OpenAPI spec

- Replace manual DOCX ZIP manipulation with poi-tl library
- Templates use {{variable}} syntax, editable in Word
- Add postProcess hook for dynamic content (table rows)
- Clean up unused generate() method from TemplateProvider
- Remove custom ExportResult/ExportFileWriter complexity
- Add docs/openapi.yaml — OpenAPI 3.1 spec for export API

pom.xml

@@ -21,6 +21,7 @@
     <properties>
         <java.version>25</java.version>
         <poi.version>5.4.0</poi.version>
+        <poi-tl.version>1.12.2</poi-tl.version>
     </properties>
 
     <dependencies>
@@ -40,6 +41,12 @@
             <version>${poi.version}</version>
         </dependency>
 
+        <dependency>
+            <groupId>com.deepoove</groupId>
+            <artifactId>poi-tl</artifactId>
+            <version>${poi-tl.version}</version>
+        </dependency>
+
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-test</artifactId>