feat: pluggable image-to-text service with OpenAI vision backend (#1038)

Adds a full-stack image description service: schema, base class,                                                                  
OpenAI backend, gateway dispatch, client APIs (sync/async REST +
websocket), tg-describe-image CLI, IAM capability, and specs.                                                                     
                                                                                                                                    
Closes #879

specs/websocket/asyncapi.yaml

@@ -43,7 +43,7 @@ info:
     - config, flow, librarian, knowledge, collection-management
 
     **Flow-Scoped Services** (require `flow` parameter, workspace from token):
-    - agent, text-completion, prompt, document-rag, graph-rag
+    - agent, text-completion, image-to-text, prompt, document-rag, graph-rag
     - embeddings, graph-embeddings, document-embeddings
     - triples, rows, nlp-query, structured-query, sparql-query, structured-diag, row-embeddings
     - text-load, document-load, mcp-tool

specs/websocket/components/messages/ServiceRequest.yaml

@@ -24,6 +24,7 @@ payload:
     - $ref: './requests/DocumentRagRequest.yaml'
     - $ref: './requests/GraphRagRequest.yaml'
     - $ref: './requests/TextCompletionRequest.yaml'
+    - $ref: './requests/ImageToTextRequest.yaml'
     - $ref: './requests/PromptRequest.yaml'
     - $ref: './requests/EmbeddingsRequest.yaml'
     - $ref: './requests/McpToolRequest.yaml'

specs/websocket/components/messages/requests/ImageToTextRequest.yaml

@@ -0,0 +1,28 @@
+type: object
+description: WebSocket request for image-to-text service (flow-scoped service)
+required:
+  - id
+  - service
+  - flow
+  - request
+properties:
+  id:
+    type: string
+    description: Unique request identifier
+  service:
+    type: string
+    const: image-to-text
+    description: Service identifier for image-to-text service
+  flow:
+    type: string
+    description: Flow ID
+  request:
+    $ref: '../../../../api/components/schemas/image-to-text/ImageToTextRequest.yaml'
+examples:
+  - id: req-1
+    service: image-to-text
+    flow: my-flow
+    request:
+      image: iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==
+      mime_type: image/png
+      prompt: Describe this image

specs/websocket/components/schemas/RequestEnvelope.yaml

@@ -25,9 +25,10 @@ properties:
 
       Global services: iam
       Workspace-scoped services: config, flow, librarian, knowledge, collection-management
-      Flow-scoped services: agent, text-completion, prompt, document-rag, graph-rag,
-      embeddings, graph-embeddings, document-embeddings, triples, objects,
-      nlp-query, structured-query, structured-diag, text-load, document-load, mcp-tool
+      Flow-scoped services: agent, text-completion, image-to-text, prompt,
+      document-rag, graph-rag, embeddings, graph-embeddings, document-embeddings,
+      triples, objects, nlp-query, structured-query, structured-diag, text-load,
+      document-load, mcp-tool
     examples:
       - config
       - agent