## What this PR does
Aligns MongoDB configuration with postgres and mysql patterns by
introducing a separate `databases` section.
**Before:**
```yaml
users:
myuser:
db: mydb
roles:
- name: readWrite
db: mydb
- name: dbAdmin
db: mydb
```
**After:**
```yaml
users:
myuser: {}
databases:
mydb:
roles:
admin:
- myuser
readonly:
- reader
```
Changes:
- Add separate `databases` section with `roles.admin` and
`roles.readonly`
- Simplify `users` to only contain optional password field
- All users authenticate via `admin` database (MongoDB production best
practice)
- `admin` role maps to `readWrite` + `dbAdmin` permissions
- `readonly` role maps to `read` permission
## Migration
Includes migration script (`migrations/24`) that automatically converts
existing MongoDB HelmReleases:
- Detects old format users (with `db` and `roles` fields)
- Converts `readWrite`/`dbAdmin` roles → `databases.{db}.roles.admin`
- Converts `read` role → `databases.{db}.roles.readonly`
- Preserves user passwords in new `users` format
- Skips HelmReleases that don't have old format
### Release note
```release-note
[mongodb] Unified users and databases configuration to match postgres and mysql patterns. Users now defined separately from databases, with role assignments in the databases section. Includes automatic migration for existing deployments.
```
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