feat: add settings API with key-value store

GET/PUT endpoints for app settings backed by a settings table.

src/db.rs

@@ -199,6 +199,15 @@ impl Database {
         .execute(&self.pool)
         .await?;
 
+        sqlx::query(
+            "CREATE TABLE IF NOT EXISTS settings (
+                key TEXT PRIMARY KEY,
+                value TEXT NOT NULL
+            )"
+        )
+        .execute(&self.pool)
+        .await?;
+
         Ok(())
     }
 }