feat: add /admin/templates/preview endpoint; add preview button to template editor

app/admin.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Request, Form, Depends, HTTPException
+from fastapi import APIRouter, Request, Form, Depends, HTTPException, Body
 from fastapi.responses import HTMLResponse, RedirectResponse, JSONResponse
 from fastapi.templating import Jinja2Templates
 from sqlalchemy.orm import Session, joinedload, aliased
@@ -7,6 +7,7 @@ import json
 from app.db import SessionLocal, Target, NotificationChannel, MessageTemplate, WebhookEndpoint, RequestLog, DeliveryLog, ProcessingRule, RuleAction
 from app.services.stats import stats_service
 from app.services.engine import engine
+from app.templates import safe_render
 
 router = APIRouter(prefix="/admin", tags=["admin"])
 templates = Jinja2Templates(directory="templates")
@@ -424,6 +425,31 @@ async def delete_template(id: int = Form(...), db: Session = Depends(get_db)):
     db.commit()
     return RedirectResponse(url="/admin/templates", status_code=303)
 
+
+@router.post("/templates/preview")
+async def preview_template(data: dict = Body(...), db: Session = Depends(get_db)):
+    """
+    Preview a template with an optional sample payload.
+    Request JSON: { "template_content": "...", "sample_payload": {...}, "vars": {...} }
+    """
+    template_content = data.get("template_content")
+    if not template_content:
+        return JSONResponse({"error": "template_content is required"}, status_code=400)
+
+    sample_payload = data.get("sample_payload") or {}
+    # Build render context using engine's flatten helper if available
+    try:
+        render_context = engine._flatten_payload(sample_payload) if hasattr(engine, "_flatten_payload") else dict(sample_payload)
+        # Merge any provided template vars
+        extra_vars = data.get("vars") or {}
+        if isinstance(extra_vars, dict):
+            render_context.update(extra_vars)
+
+        rendered = safe_render(template_content, render_context)
+        return JSONResponse({"rendered": rendered})
+    except Exception as e:
+        return JSONResponse({"error": str(e)}, status_code=400)
+
 # --- Logs ---
 @router.get("/logs", response_class=HTMLResponse)
 async def list_logs(request: Request, db: Session = Depends(get_db)):