Add 7 ADRs documenting backend architecture decisions

docs/decisions/0002-database-migration-mongodb-to-mysql.md

@@ -0,0 +1,27 @@
+# ADR 0002: Database Migration — MongoDB to MySQL with Sequelize
+
+**Date:** 2026-04-26
+**Status:** Accepted
+
+## Context
+
+The project initially used MongoDB (evidence: `mongo:start`/`mongo:stop` Docker scripts still present in `package.json`, and `$oid` ObjectId references in the original data export). MongoDB was likely chosen for its flexible schema during early prototyping.
+
+As the data model stabilised and relational queries became more common (joins between users, jumps, canopies, drop zones, etc.), a relational database became a better fit. MySQL is a well-known, widely hosted relational database with strong Sequelize support.
+
+## Decision
+
+Migrate to MySQL with Sequelize as the ORM. Sequelize provides:
+- Model definitions with typed fields
+- Migration system (`sequelize-cli db:migrate`) for schema versioning
+- Seeders for initial reference data
+- Relationship declarations (`src/database/relationships/`)
+
+The `mongo:start`/`mongo:stop` npm scripts are legacy artefacts and can be removed when confirmed no longer needed.
+
+## Consequences
+
+- **Positive:** Relational integrity enforced at the database level. Joins are first-class.
+- **Positive:** Sequelize migrations provide a reproducible setup path (`npm run setup:api`).
+- **Negative:** Less flexible schema than MongoDB — changes require migrations.
+- **Note:** The original data was exported from MongoDB (documents with `$oid` fields) and re-imported into MySQL via a one-shot migration script. That script has since been removed from the frontend codebase.