fix(storage): make openelements.storage.s3.region optional

[herbie-bot]

Why

The region belongs to the request signature, not to the address. SigV4 needs some string there, but only AWS derives meaning from it — on AWS it must match the bucket's region, while Hetzner Object Storage and other S3-compatible providers accept whatever is sent and ignore it.

Requiring it made a Hetzner deployment configure a value nobody reads, and failed start-up over its absence.

What changes

openelements.storage.s3.region is now optional, defaulting to us-east-1 — the value S3-compatible providers conventionally accept and ignore. Endpoint, bucket and credentials stay required, and their absence still fails start-up naming the property.

openelements.storage.s3.endpoint=https://fsn1.your-objectstorage.com   # required
openelements.storage.s3.bucket=my-objects                              # required
openelements.storage.s3.access-key=${S3_ACCESS_KEY}                    # required
openelements.storage.s3.secret-key=${S3_SECRET_KEY}                    # required
openelements.storage.s3.region=eu-central-1                            # optional

The default lives in StorageProperties.S3.DEFAULT_REGION and is applied by the binder via @DefaultValue, so the record component stays non-null and Region.of(...) has nothing to guard against.

Tests

Two added: type=s3 without a region starts and resolves to the default; a configured region survives. The incomplete-block test now expects bucket rather than region as the first missing property — which is the behaviour change, stated as an assertion.

93 tests in the module; ./mvnw -Pfull-build clean verify -Dmaven.javadoc.failOnWarnings=true green across all twelve.

README and docs/releases/upgrade-to-1.5.md updated, with the reason spelled out rather than just the flag flipped.

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