collab: Limit free tier usage across all models (#25573)

This PR adjusts the usage checks for the LLM free tier.

Previously we would limit the usage on a per-model basis, meaning the
user would get $10/mo free for each model they had access to.

We now have usage for all models count towards the free tier limit.

Release Notes:

- N/A

crates/collab/src/llm.rs

@@ -451,19 +451,19 @@ async fn check_usage_limit(
         return Ok(());
     }
 
+    let user_id = UserId::from_proto(claims.user_id);
     let model = state.db.model(provider, model_name)?;
     let usage = state
         .db
-        .get_usage(
-            UserId::from_proto(claims.user_id),
-            provider,
-            model_name,
-            Utc::now(),
-        )
+        .get_usage(user_id, provider, model_name, Utc::now())
         .await?;
     let free_tier = claims.free_tier_monthly_spending_limit();
 
-    if usage.spending_this_month >= free_tier {
+    let spending_this_month = state
+        .db
+        .get_user_spending_for_month(user_id, Utc::now())
+        .await?;
+    if spending_this_month >= free_tier {
         if !claims.has_llm_subscription {
             return Err(Error::http(
                 StatusCode::PAYMENT_REQUIRED,