Refactored `NewIdentityAccessManagementWithStore` to remove mutual
exclusivity between static (file-based) and dynamic (filer-based)
configuration loading.
Previously, if a static config configuration was present (including the
legacy `IamConfig` option used by `weed mini`), it prevented loading
users from the filer.
Now, the system loads the static configuration first (if present), and
then *always* attempts to merge in the dynamic configuration from the
filer. This ensures that:
1. Static users (e.g. from `weed mini` env vars or `-s3.config`) are loaded and protected.
2. Dynamic users (e.g. created via Admin UI and stored in Filer) are also loaded and available.
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