PDF Text editor (#4724)

## Summary
- add a `PdfJsonConversionService` that serializes PDF text, fonts, and
metadata to JSON and rebuilds a PDF from the same structure
- expose REST endpoints for `/pdf/json` and `/json/pdf` conversions
using the existing convert API infrastructure
- define JSON model classes capturing document metadata, font
information, and positioned text elements

## Testing
- `./gradlew spotlessApply` *(fails: plugin
org.springframework.boot:3.5.4 unavailable in build environment)*
- `./gradlew build` *(fails: plugin org.springframework.boot:3.5.4
unavailable in build environment)*

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https://chatgpt.com/codex/tasks/task_b_68f8e98d94ac8328a0e499e541528b6f

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Co-authored-by: EthanHealy01 <ethan.healy.21@gmail.com>

docker/compose/docker-compose.yml

@@ -5,10 +5,6 @@ services:
       dockerfile: docker/backend/Dockerfile
     container_name: stirling-pdf-backend
     restart: on-failure:5
-    deploy:
-      resources:
-        limits:
-          memory: 4G
     healthcheck:
       test: ["CMD-SHELL", "curl -f http://localhost:8080/api/v1/info/status | grep -q 'UP'"]
       interval: 5s