feat: add POST /ai/session-to-flow endpoint (Task 21)

Converts a completed session into a reusable procedural flow using AI.
Includes quota checking, usage recording, and proper error handling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/ai_builder.py

@@ -6,6 +6,9 @@
   POST /ai/branch-detail — Stage 3: AI generates detail for one branch
   POST /ai/assemble      — Stage 4: assemble branches into tree (no AI)
   GET  /ai/quota         — quota status
+
+Session conversion:
+  POST /ai/session-to-flow — Convert a completed session into a procedural flow
 """
 import logging
 from typing import Annotated
@@ -40,6 +43,8 @@ from app.schemas.ai_builder import (
     AIAssembleResponse,
     AIQuotaStatusResponse,
 )
+from app.schemas.session_to_flow import SessionToFlowRequest, SessionToFlowResponse
+from app.services.session_to_flow_service import generate_flow_from_session
 
 logger = logging.getLogger(__name__)
 
@@ -437,3 +442,97 @@ async def assemble(
         summary=stats,
         status="completed",
     )
+
+
+@router.post("/session-to-flow", response_model=SessionToFlowResponse)
+@limiter.limit("5/minute")
+async def session_to_flow(
+    request: Request,
+    data: SessionToFlowRequest,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+    _: None = Depends(require_engineer_or_admin),
+):
+    """Convert a completed troubleshooting session into a reusable procedural flow."""
+    _require_ai_enabled()
+
+    # Check AI quota
+    allowed, quota_status = await check_ai_quota(
+        user_id=current_user.id,
+        account_id=current_user.account_id,
+        db=db,
+        billing_anchor=current_user.ai_billing_cycle_anchor_at,
+        is_super_admin=current_user.is_super_admin,
+    )
+    if not allowed:
+        reset_key = (
+            "daily_reset_at"
+            if quota_status.get("deny_reason") == "daily"
+            else "monthly_reset_at"
+        )
+        raise HTTPException(
+            status_code=status.HTTP_429_TOO_MANY_REQUESTS,
+            detail={
+                "message": f"AI build limit exceeded ({quota_status['deny_reason']})",
+                "reset_at": quota_status.get(reset_key),
+                "quota": quota_status,
+            },
+        )
+
+    plan = await get_user_plan(current_user.account_id, db)
+
+    try:
+        result = await generate_flow_from_session(
+            session_id=data.session_id,
+            user_id=current_user.id,
+            account_id=current_user.account_id,
+            db=db,
+        )
+    except ValueError as e:
+        logger.warning("session_to_flow validation error: %s", e)
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=str(e),
+        )
+    except Exception as e:
+        logger.exception("session_to_flow failed: %s", e)
+        await record_ai_usage(
+            user_id=current_user.id,
+            account_id=current_user.account_id,
+            conversation_id=None,
+            generation_type="session_to_flow",
+            tier=plan,
+            input_tokens=0,
+            output_tokens=0,
+            estimated_cost=0,
+            succeeded=False,
+            counts_toward_quota=False,
+            error_code=type(e).__name__,
+            extra_data={"session_id": data.session_id, "error": str(e)},
+            db=db,
+        )
+        await db.commit()
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"Failed to generate flow: {type(e).__name__}. Please try again.",
+        )
+
+    # Record successful quota-consuming usage
+    await record_ai_usage(
+        user_id=current_user.id,
+        account_id=current_user.account_id,
+        conversation_id=None,
+        generation_type="session_to_flow",
+        tier=plan,
+        input_tokens=0,
+        output_tokens=0,
+        estimated_cost=0,
+        succeeded=True,
+        counts_toward_quota=True,
+        error_code=None,
+        extra_data={"session_id": data.session_id},
+        db=db,
+    )
+    await db.commit()
+
+    return SessionToFlowResponse(**result)