feat: add POST /ai/fix-tree endpoint for AI-powered validation fixes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/ai_fix.py

@@ -0,0 +1,78 @@
+"""AI auto-fix endpoint for tree validation errors.
+
+POST /ai/fix-tree — accepts a tree with validation errors and returns
+AI-generated fix proposals for each error.
+"""
+
+import logging
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, HTTPException, Request, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.api.deps import get_db, require_engineer_or_admin
+from app.core.config import settings
+from app.core.rate_limit import limiter
+from app.core.ai_fix_service import generate_fixes
+from app.models.user import User
+from app.schemas.ai_fix import (
+    AIFixTreeRequest,
+    AIFixTreeResponse,
+    AIFixProposal,
+    AIFixTokenUsage,
+)
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/ai", tags=["ai-fix"])
+
+
+def _require_ai_enabled() -> None:
+    """Raise 503 if AI is not configured."""
+    if not settings.ai_enabled:
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail="AI fix is not configured. Set GOOGLE_AI_API_KEY or ANTHROPIC_API_KEY.",
+        )
+
+
+@router.post("/fix-tree", response_model=AIFixTreeResponse)
+@limiter.limit("10/minute")
+async def fix_tree(
+    request: Request,
+    body: AIFixTreeRequest,
+    user: Annotated[User, Depends(require_engineer_or_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+):
+    """Generate AI-powered fixes for tree validation errors."""
+    _require_ai_enabled()
+
+    validation_errors = [
+        {"node_id": e.node_id, "message": e.message}
+        for e in body.validation_errors
+    ]
+
+    try:
+        fixes, input_tokens, output_tokens = await generate_fixes(
+            tree_structure=body.tree_structure,
+            tree_name=body.tree_name,
+            tree_type=body.tree_type,
+            validation_errors=validation_errors,
+        )
+    except RuntimeError as exc:
+        logger.error("AI provider not available: %s", exc)
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail=str(exc),
+        )
+    except Exception as exc:
+        logger.exception("Unexpected error in AI fix service")
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="An unexpected error occurred while generating fixes.",
+        )
+
+    return AIFixTreeResponse(
+        fixes=[AIFixProposal(**f) for f in fixes],
+        tokens_used=AIFixTokenUsage(input=input_tokens, output=output_tokens),
+    )