collab: Setup database for LLM service (#15882)

This PR puts the initial infrastructure for the LLM service's database
in place.

The LLM service will be using a separate Postgres database, with its own
set of migrations.

Currently we only connect to the database in development, as we don't
yet have the database setup for the staging/production environments.

Release Notes:

- N/A

crates/collab/src/llm/db/tables/provider.rs

@@ -0,0 +1,26 @@
+use sea_orm::entity::prelude::*;
+
+use crate::llm::db::ProviderId;
+
+/// An LLM provider.
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel)]
+#[sea_orm(table_name = "providers")]
+pub struct Model {
+    #[sea_orm(primary_key)]
+    pub id: ProviderId,
+    pub name: String,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {
+    #[sea_orm(has_many = "super::model::Entity")]
+    Models,
+}
+
+impl Related<super::model::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::Models.def()
+    }
+}
+
+impl ActiveModelBehavior for ActiveModel {}