Add first-class ChatGPT subscription provider support (#881)

* Add first-class ChatGPT subscription provider support

* Address PR feedback: move uuid import to top, reuse parsed config in up()

* Add ChatGPT token watchdog for seamless long-lived sessions

* Address PR feedback: error on stream=false for ChatGPT, fix auth file permissions

* Replace ChatGPT watchdog/restart with passthrough_auth

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Co-authored-by: Musa Malik <musam@uw.edu>

config/plano_config_schema.yaml

@@ -190,7 +190,13 @@ properties:
             - openai
             - xiaomi
             - gemini
+            - chatgpt
             - digitalocean
+        headers:
+          type: object
+          additionalProperties:
+            type: string
+          description: "Additional headers to send with upstream requests (e.g., ChatGPT-Account-Id, originator)."
         routing_preferences:
           type: array
           items:
@@ -239,7 +245,13 @@ properties:
             - openai
             - xiaomi
             - gemini
+            - chatgpt
             - digitalocean
+        headers:
+          type: object
+          additionalProperties:
+            type: string
+          description: "Additional headers to send with upstream requests (e.g., ChatGPT-Account-Id, originator)."
         routing_preferences:
           type: array
           items: