collab: Add usages table to LLM database (#15884)

This PR adds a `usages` table to the LLM database.

We'll use this to track usage for rate-limiting purposes.

Release Notes:

- N/A

crates/collab/src/llm/db/queries/usages.rs

@@ -0,0 +1,57 @@
+use rpc::LanguageModelProvider;
+
+use super::*;
+
+impl LlmDatabase {
+    pub async fn find_or_create_usage(
+        &self,
+        user_id: i32,
+        provider: LanguageModelProvider,
+        model_name: &str,
+    ) -> Result<usage::Model> {
+        self.transaction(|tx| async move {
+            let provider_name = match provider {
+                LanguageModelProvider::Anthropic => "anthropic",
+                LanguageModelProvider::OpenAi => "open_ai",
+                LanguageModelProvider::Google => "google",
+                LanguageModelProvider::Zed => "zed",
+            };
+
+            let model = model::Entity::find()
+                .inner_join(provider::Entity)
+                .filter(
+                    provider::Column::Name
+                        .eq(provider_name)
+                        .and(model::Column::Name.eq(model_name)),
+                )
+                .one(&*tx)
+                .await?
+                // TODO: Create the model, if one doesn't exist.
+                .ok_or_else(|| anyhow!("no model found for {provider_name}:{model_name}"))?;
+            let model_id = model.id;
+
+            let existing_usage = usage::Entity::find()
+                .filter(
+                    usage::Column::UserId
+                        .eq(user_id)
+                        .and(usage::Column::ModelId.eq(model_id)),
+                )
+                .one(&*tx)
+                .await?;
+            if let Some(usage) = existing_usage {
+                return Ok(usage);
+            }
+
+            let usage = usage::Entity::insert(usage::ActiveModel {
+                user_id: ActiveValue::set(user_id),
+                model_id: ActiveValue::set(model_id),
+                ..Default::default()
+            })
+            .exec_with_returning(&*tx)
+            .await?;
+
+            Ok(usage)
+        })
+        .await
+    }
+}