docs: add ADR for database migration squash strategy

Author: dlachaume

docs/website/adr/009-database-migration-squashing.md

@@ -0,0 +1,30 @@
+---
+slug: 9
+title: |
+  9. Database migration squashing
+authors:
+  - name: Mithril Team
+tags: [Accepted]
+date: 2025-03-13
+---
+
+## Status
+
+Accepted
+
+## Context
+
+Many database migrations have accumulated over time for the Mithril nodes. As the migrations are done sequentially, the resulting scheme of the database is hard to understand and maintain.
+
+## Decision
+
+To mitigate these concerns, we have decided to implement a migration squashing once we have stacked too many migrations for a store: all the existing migrations are consolidated and replaced by the equivalent new migration.
+
+## Consequences
+
+- This applies to the migrations of all the stores of the Mithril nodes
+- A squashed migration will be applied when a database is initialized for the first time
+- A squashed migration must be optional and run only if not already applied previously with the equivalent migration sequence
+- Some nodes may have only partially applied the equivalent sequence of migrations and thus can not apply the squashed migration right away:
+  - They need to first run the migration with the latest distribution that does not ship the squashed migration so that their database is ready to play the squashed migration
+  - This distribution is associated to a squashed migration to provide a smooth user experience.