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

.env.example

@@ -26,6 +26,13 @@
 
 AI_PROVIDER=replicate
 
+# Per-operation provider overrides (optional — blank means use AI_PROVIDER above)
+# Example: use OpenAI for text-to-image (best quality) but InvokeAI for everything else
+#AI_PROVIDER_TXT2IMG=openai
+#AI_PROVIDER_INPAINT=invokeai
+#AI_PROVIDER_IMG2IMG=invokeai
+#AI_PROVIDER_OUTPAINT=invokeai
+
 
 # =============================================================================
 # STEP 2: Get Your API Key