collab: Update billing code for LLM usage billing (#18879)

This PR reworks our existing billing code in preparation for charging
based on LLM usage.

We aren't yet exercising the new billing-related code outside of
development.

There are some noteworthy changes for our existing LLM usage tracking:

- A new `monthly_usages` table has been added for tracking usage
per-user, per-model, per-month
- The per-month usage measures have been removed, in favor of the
`monthly_usages` table
- All of the per-month metrics in the Clickhouse rows have been changed
from a rolling 30-day window to a calendar month

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/collab/src/main.rs

@@ -6,6 +6,7 @@ use axum::{
     routing::get,
     Extension, Router,
 };
+use collab::api::billing::sync_llm_usage_with_stripe_periodically;
 use collab::api::CloudflareIpCountryHeader;
 use collab::llm::{db::LlmDatabase, log_usage_periodically};
 use collab::migrations::run_database_migrations;
@@ -29,7 +30,7 @@ use tower_http::trace::TraceLayer;
 use tracing_subscriber::{
     filter::EnvFilter, fmt::format::JsonFields, util::SubscriberInitExt, Layer,
 };
-use util::ResultExt as _;
+use util::{maybe, ResultExt as _};
 
 const VERSION: &str = env!("CARGO_PKG_VERSION");
 const REVISION: Option<&'static str> = option_env!("GITHUB_SHA");
@@ -136,6 +137,28 @@ async fn main() -> Result<()> {
                     fetch_extensions_from_blob_store_periodically(state.clone());
                     spawn_user_backfiller(state.clone());
 
+                    let llm_db = maybe!(async {
+                        let database_url = state
+                            .config
+                            .llm_database_url
+                            .as_ref()
+                            .ok_or_else(|| anyhow!("missing LLM_DATABASE_URL"))?;
+                        let max_connections = state
+                            .config
+                            .llm_database_max_connections
+                            .ok_or_else(|| anyhow!("missing LLM_DATABASE_MAX_CONNECTIONS"))?;
+
+                        let mut db_options = db::ConnectOptions::new(database_url);
+                        db_options.max_connections(max_connections);
+                        LlmDatabase::new(db_options, state.executor.clone()).await
+                    })
+                    .await
+                    .trace_err();
+
+                    if let Some(llm_db) = llm_db {
+                        sync_llm_usage_with_stripe_periodically(state.clone(), llm_db);
+                    }
+
                     app = app
                         .merge(collab::api::events::router())
                         .merge(collab::api::extensions::router())