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]>

app/common/build.gradle

@@ -37,6 +37,8 @@ dependencies {
     api 'com.drewnoakes:metadata-extractor:2.19.0' // Image metadata extractor
     api 'com.vladsch.flexmark:flexmark-html2md-converter:0.64.8'
     api "org.apache.pdfbox:pdfbox:$pdfboxVersion"
+    api "org.apache.pdfbox:xmpbox:$pdfboxVersion"
+    api "org.apache.pdfbox:preflight:$pdfboxVersion"
     api 'com.github.junrar:junrar:7.5.5' // RAR archive support for CBR files
     api 'jakarta.servlet:jakarta.servlet-api:6.1.0'
     api 'org.snakeyaml:snakeyaml-engine:2.10'