Added backend patterns.md (#21782)

As discussed at backend sync
https://us-65885.app.gong.io/call?id=8041045095900447703

server/docs/patterns.md

@@ -0,0 +1,26 @@
+# Backend patterns
+
+The backend software patterns that we follow in Fleet.
+
+> NOTE: There are always exceptions to the rules, but we try to follow these patterns as much as possible unless a specific use case calls
+> for something else. These should be discussed within the team and documented before merging.
+
+## MySQL
+
+Use high precision for all time fields. Precise timestamps make sure that we can accurately track when records were created and updated,
+keep records in order with a reliable sort, and speed up testing by not having to wait for the time to
+update. [MySQL reference](https://dev.mysql.com/doc/refman/8.4/en/date-and-time-type-syntax.html). [Backend sync where discussed](https://us-65885.app.gong.io/call?id=8041045095900447703).
+Example:
+
+```sql
+CREATE TABLE `sample` (
+  `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
+  `created_at` TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6),
+  `updated_at` TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),
+  PRIMARY KEY (`id`)
+);
+```
+
+Do not use [goqu](https://github.com/doug-martin/goqu); use MySQL queries directly. Searching for, understanding, and debugging direct MySQL
+queries is easier. If needing to modify an existing `goqu` query, try to rewrite it in
+MySQL. [Backend sync where discussed](https://us-65885.app.gong.io/call?id=8041045095900447703).