Default PaintPlus OpenAI provider to gpt-image-2, fix GPU quick-start

dall-e-2/dall-e-3 retired May 12 2026 and gpt-image-1 deprecates Oct 23
2026, so move every OpenAI default (config.py, both provider classes,
both compose files, .env.example, the in-app provider-settings dropdown,
README) to gpt-image-2 for both generation and edits. Also fix response
parsing in ai_provider.py's OpenAIProvider, which never sent a model
param and assumed a url response — gpt-image-1/2 only return b64_json.

Separately, .env.example shipped AI_PROVIDER=replicate by default, but
replicate has no driver in remote_provider.py, so following the
documented "cp .env.example .env" setup silently broke every AI call
and defeated the GPU quick-start (an explicit non-empty .env value
overrides docker-compose.gpu.yml's own local_gpu fallback). Default to
local_gpu instead, mark replicate/stability as not-yet-implemented, and
recommend Lykon/dreamshaper-8-inpainting as a hands/face-tuned
HF_MODEL_INPAINT override for 4-6GB cards (Quadro P2200, GTX 1060/1660).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nb2vJ8W7bHKx1JXVvpCraH

paintplus/backend/app/config.py

@@ -27,8 +27,8 @@ class Settings(BaseSettings):
 
     # Provider API Keys
     openai_api_key: str = ""
-    openai_model: str = "dall-e-3"          # text-to-image (generations endpoint)
-    openai_edit_model: str = "gpt-image-1"  # inpaint/img2img/outpaint (edits endpoint — dall-e-3 isn't supported there)
+    openai_model: str = "gpt-image-2"       # text-to-image (generations endpoint)
+    openai_edit_model: str = "gpt-image-2"  # inpaint/img2img/outpaint (edits endpoint)
     stability_api_key: str = ""
     replicate_api_key: str = ""