add /health/db endpoint for supabase connection diagnostics

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

python-backend/api/main.py

@@ -165,6 +165,24 @@ def health():
     return {"status": "ok", "service": "signal-api", "version": "1.0.0"}
 
 
+@app.get("/health/db")
+def health_db():
+    from core.supabase_client import get_client
+    import os
+    client = get_client()
+    if not client:
+        return {
+            "status": "unavailable",
+            "supabase_url_set": bool(os.getenv("SUPABASE_URL")),
+            "service_key_set": bool(os.getenv("SUPABASE_SERVICE_KEY")),
+        }
+    try:
+        result = client.table("organizations").select("id").limit(1).execute()
+        return {"status": "ok", "org_count": len(result.data)}
+    except Exception as e:
+        return {"status": "error", "detail": str(e)}
+
+
 @app.post("/api/upload", response_model=UploadResponse)
 async def upload_csv(
     file: UploadFile = File(...),