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 <[email protected]>

docker/backend/Dockerfile.ultra-lite

@@ -60,7 +60,9 @@ RUN echo "@testing https://dl-cdn.alpinelinux.org/alpine/edge/main" | tee -a /et
         curl \
         shadow \
         su-exec \
-        openjdk21-jre && \
+        openjdk21-jre \
+        ghostscript \
+        fontforge && \
     # User permissions
     mkdir -p /configs /logs /customFiles /usr/share/fonts/opentype/noto /tmp/stirling-pdf /pipeline/watchedFolders /pipeline/finishedFolders && \
     chmod +x /scripts/*.sh && \