⬆️(backend) upgrade docspec to v3.0.0 and adapt converter API

Summary

- Bump docspec Docker image from `2.6.3` to `3.0.0` and adapt
`DocSpecConverter` to the new API (raw body upload with explicit
`Content-Type`/`Accept` headers instead of multipart form)

Important

**The Docker image (`ghcr.io/docspecio/api:3.0.0`) must be updated
alongside the code changes.** The new request format is incompatible
with v2.x — deploying only the code without updating the image (or vice
versa) will break document conversion.

src/helm/env.d/dev/values.impress.yaml.gotmpl

@@ -178,7 +178,7 @@ docSpec:
   image:
     repository: ghcr.io/docspecio/api
     pullPolicy: IfNotPresent
-    tag: "2.6.3"
+    tag: "3.0.1"
 
   probes:
     liveness:

src/helm/env.d/feature/values.impress.yaml.gotmpl

@@ -154,7 +154,7 @@ docSpec:
   image:
     repository: ghcr.io/docspecio/api
     pullPolicy: IfNotPresent
-    tag: "2.6.3"
+    tag: "3.0.1"
 
   probes:
     liveness:

src/helm/impress/values.yaml

@@ -768,7 +768,7 @@ docSpec:
   image:
     repository: ghcr.io/docspecio/api
     pullPolicy: IfNotPresent
-    tag: "2.6.3"
+    tag: "3.0.1"
 
   ## @param docSpec.command Override the docSpec container command
   command: []