feat(db): Added database setup and basic types (#5)

Co-authored-by: Sharang Parnerkar <parnerkarsharang@gmail.com>
Reviewed-on: #5

src/infrastructure/database.rs

@@ -0,0 +1,52 @@
+//! MongoDB connection wrapper with typed collection accessors.
+
+use mongodb::{bson::doc, Client, Collection};
+
+use super::Error;
+use crate::models::{OrgBillingRecord, OrgSettings, UserPreferences};
+
+/// Thin wrapper around [`mongodb::Database`] that provides typed
+/// collection accessors for the application's domain models.
+#[derive(Clone, Debug)]
+pub struct Database {
+    inner: mongodb::Database,
+}
+
+impl Database {
+    /// Connect to MongoDB, select the given database, and verify
+    /// connectivity with a `ping` command.
+    ///
+    /// # Arguments
+    ///
+    /// * `uri` - MongoDB connection string (e.g. `mongodb://localhost:27017`)
+    /// * `db_name` - Database name to use
+    ///
+    /// # Errors
+    ///
+    /// Returns `Error::DatabaseError` if the client cannot be created
+    /// or the ping fails.
+    pub async fn connect(uri: &str, db_name: &str) -> Result<Self, Error> {
+        let client = Client::with_uri_str(uri).await?;
+        let db = client.database(db_name);
+
+        // Verify the connection is alive.
+        db.run_command(doc! { "ping": 1 }).await?;
+
+        Ok(Self { inner: db })
+    }
+
+    /// Collection for per-user preferences (theme, custom topics, etc.).
+    pub fn user_preferences(&self) -> Collection<UserPreferences> {
+        self.inner.collection("user_preferences")
+    }
+
+    /// Collection for organisation-level settings.
+    pub fn org_settings(&self) -> Collection<OrgSettings> {
+        self.inner.collection("org_settings")
+    }
+
+    /// Collection for per-cycle billing records.
+    pub fn org_billing(&self) -> Collection<OrgBillingRecord> {
+        self.inner.collection("org_billing")
+    }
+}