Fix template lookup column name in send-notification

The function was using .eq("template_key") but the actual column name
in notification_templates table is "key". This caused "Template not found"
errors even when the template existed and was active.

Changes:
- send-notification/index.ts: Changed .eq("template_key") to .eq("key")
- Matches the pattern used in send-auth-otp and other edge functions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

supabase/functions/send-notification/index.ts

@@ -215,7 +215,7 @@ serve(async (req: Request): Promise<Response> => {
     const { data: template, error: templateError } = await supabase
       .from("notification_templates")
       .select("*")
-      .eq("template_key", template_key)
+      .eq("key", template_key)
       .eq("is_active", true)
       .single();