feat: optional MySQL/MariaDB database driver for deployments that cannot use local SQLite

docs/architecture.md

@@ -80,6 +80,18 @@ bin/renewcms  ──  /v1/*  API
 
 Uploaded images are addressed by a stored name (`<uuid>.<ext>`). `CreateImageUseCase` calls `Storage.Put` (which generates the name), then `Repository.Create` with `Path = /static/uploadedImages/<name>`, and deletes the blob again if the row cannot be created. `DeleteMediaUseCase` and `MergeMediaUseCase` remove the blob first and the row second (`mediaRemover`), so a failed blob delete keeps the row and the delete can be retried. `GET /static/uploadedImages/<name>` is served by `OpenMediaUseCase` through `api/handlers/staticimages`, which uses `http.ServeContent` when the blob is seekable and streams it otherwise; `ErrBlobNotFound` maps to 404 and any other error to 500.
 
-SQL in repositories must run on SQLite, MySQL/MariaDB and PostgreSQL: substring searches use `LIKE ... ESCAPE '!'` through `containsPattern` instead of `instr`. `Settings.DataDir()` is where update state, staging and backups live; it is derived from `DB_FILE` today and does not depend on where the database is.
+SQL in repositories must run on SQLite, MySQL/MariaDB and PostgreSQL: substring searches use `LIKE ... ESCAPE '!'` through `containsPattern` instead of `instr`. `Settings.DataDir()` is where update state, staging and backups live; it is `DATA_DIR` when set and the directory of `DB_FILE` otherwise, so it does not depend on where the database is.
 
-Adding a database driver: implement `database.Driver` (`Open`, and `NewBackup` returning a `domain/update.DatabaseBackup`), register it in `drivers`, then select it in `openDatabase` in the composition root (and add the env vars to `main.go`, `.env.example` and `configuration.md`). Adding a storage backend: implement `image.Storage`, add a case to `storage.Open`, and select it in `openImageStorage`. No use case, handler or repository changes.
+Two drivers are registered in `database.drivers`: `sqlite` (default) and `mysql` (MySQL 8 and MariaDB 11, through `gorm.io/driver/mysql`, pure Go so `CGO_ENABLED=0` and the six release targets still build). `DB_DRIVER` picks one in `databaseConfig` (composition root); an unknown name fails in `database.Connect` with `unsupported database driver "<name>"`. `database.MySQLDSN` builds the DSN from separate variables, and the driver redacts the password from every error and names only host and port. `Connect` applies the optional `PoolSettings` to any driver. What keeps the repositories identical on both engines, all without touching the domain, use cases or handlers:
+
+- Indexed and unique strings carry `size:` tags in the models (a MySQL index needs a bounded `varchar`). SQLite maps them to the same `text` column, so an existing SQLite file migrates with no schema or row change (`test/sqlite_schema_unchanged_test.go` builds a file from the old model definitions and compares `sqlite_master` and every row).
+- `Migrate` creates MySQL tables with `utf8mb4_bin` (`gorm:table_options`), so `Bob` and `bob` are distinct like in SQLite. The DSN also sets `collation=utf8mb4_bin`.
+- The DSN sets `clientFoundRows=true`: an update that matches a row but changes nothing still reports one affected row, as in SQLite. `InviteRepository.Renew`, `deleteOrNotFound` and `PurgeSettingsRepository.SaveLastRun` depend on that.
+- The Go driver sends a zero `time.Time` as `0000-00-00`, which MySQL's default `NO_ZERO_DATE` rejects, so each session sets `sql_mode` to the server default minus `NO_ZERO_DATE`. Reading `0000-00-00` gives the zero time back.
+- `UserRepository.Create` returns the filled model instead of re-reading it, because MySQL has no `RETURNING`.
+
+Backups are a `domain/update.DatabaseBackup` per driver. SQLite uses `VACUUM INTO`. MySQL uses `DumpBackup`, a pure Go logical dump (`SHOW CREATE TABLE` plus every row, strings escaped and binary values as hex, NULL and zero dates kept) written inside one repeatable-read transaction to `DATA_DIR/backups/renewcms-<time>.sql.partial` and renamed once complete. Any failure removes the partial file and returns an error, so the update download fails instead of claiming a backup. Both keep the newest 3 files (`pruneBackups`).
+
+`task test` runs everything that needs no server. The repository contract tests (`test/repository_contract_test.go`) run on SQLite always, and on MySQL when `RENEWCMS_TEST_MYSQL_DSN` points at a server (admin DSN; each test creates and drops its own database). `task test:mysql` starts `mysql:8.4` and then `mariadb:11` in Docker and runs the `MySQL` tests against each.
+
+Adding a database driver: implement `database.Driver` (`Open`, and `NewBackup` returning a `domain/update.DatabaseBackup`), register it in `drivers`, then select it in `databaseConfig` in the composition root (and add the env vars to `main.go`, `.env.example` and `configuration.md`). Adding a storage backend: implement `image.Storage`, add a case to `storage.Open`, and select it in `openImageStorage`. No use case, handler or repository changes.