Add per-operation AI provider routing

Each operation (inpaint, txt2img, img2img, outpaint) can now use a different
provider. Resolution order: per-op override → global AI_PROVIDER default.

Example: txt2img→openai, inpaint→invokeai, everything else→invokeai default.

Backend:
- config.py: add AI_PROVIDER_INPAINT / TXT2IMG / IMG2IMG / OUTPAINT settings
- remote_provider.py: get_remote_provider(operation) resolves override then default;
  _build_provider() extracted as shared factory; _OP_FIELD maps op→setting name
- ai_tools.py: each endpoint passes its operation to _require_remote();
  GET /api/config runs per-op health checks concurrently, returns operations map
  and overrides; POST /api/config accepts and applies per-op override fields

Frontend:
- ai_provider_settings.js: four new selects (inpaint/txt2img/img2img/outpaint);
  persists to localStorage and sends per-op fields to POST /api/config
- provider-badge.js: shows override summary (e.g. "invokeai · txt2img→openai")
  and per-op health in tooltip
- .env.example: document per-op override env vars with examples

https://claude.ai/code/session_01B58MaJCU1R6KwBDJCp8AfN

backend/app/config.py

@@ -13,9 +13,18 @@ class Settings(BaseSettings):
 
     # AI Provider
     # Local: blank or "mock" — always available, no config needed
-    # Remote (set ONE): openai | invokeai | comfyui | replicate | stability
+    # Remote default (used for any operation without a specific override):
+    #   openai | invokeai | comfyui | replicate | stability
     ai_provider: str = "mock"
 
+    # Per-operation provider overrides — blank means use ai_provider default.
+    # Operations: inpaint, txt2img, img2img, outpaint
+    # Example: AI_PROVIDER_TXT2IMG=openai  (use OpenAI for text-to-image only)
+    ai_provider_inpaint: str = ""   # remote inpaint / replace selection
+    ai_provider_txt2img: str = ""   # text-to-image
+    ai_provider_img2img: str = ""   # image-to-image
+    ai_provider_outpaint: str = ""  # expand canvas
+
     # Provider API Keys
     openai_api_key: str = ""
     openai_model: str = "dall-e-3"