docs(libsql-server): fix LIBSQL_BOTTOMLESS_AWS_DEFAULT_REGION env var name

The README documented the bottomless region environment variable as
LIBSQL_BOTTOMLESS_AWS_REGION, but the replicator source code
(bottomless/src/replicator.rs:139,201) actually reads
LIBSQL_BOTTOMLESS_AWS_DEFAULT_REGION.

Setting the documented name results in sqld failing with:
  Error: Internal Error: LIBSQL_BOTTOMLESS_AWS_DEFAULT_REGION was not set

This corrects the README to match the actual implementation, adds
LIBSQL_BOTTOMLESS_AWS_SESSION_TOKEN (also supported by the replicator
but undocumented), improves the comment descriptions, and adds a note
with the exact error message users will encounter so they can search
for it.

Verified against bottomless/src/replicator.rs env_var calls.

Author: Booki-Brain

libsql-server/README.md

@@ -89,11 +89,16 @@ environment variables can be used to configure the replication:
 ```bash
 LIBSQL_BOTTOMLESS_BUCKET=my-bucket                 # Default bucket name: bottomless
 LIBSQL_BOTTOMLESS_ENDPOINT='http://localhost:9000' # address can be overridden for local testing, e.g. with Minio
-LIBSQL_BOTTOMLESS_AWS_SECRET_ACCESS_KEY=           # regular AWS variables are used
-LIBSQL_BOTTOMLESS_AWS_ACCESS_KEY_ID=               # ... to set up auth, regions, etc.
-LIBSQL_BOTTOMLESS_AWS_REGION=                      # .
+LIBSQL_BOTTOMLESS_AWS_ACCESS_KEY_ID=               # AWS access key ID (or S3-compatible provider)
+LIBSQL_BOTTOMLESS_AWS_SECRET_ACCESS_KEY=           # AWS secret access key (or S3-compatible provider)
+LIBSQL_BOTTOMLESS_AWS_DEFAULT_REGION=us-east-1     # AWS region (required; use us-east-1 for MinIO/local dev)
+LIBSQL_BOTTOMLESS_AWS_SESSION_TOKEN=               # Optional, for temporary AWS credentials
 ```
 
+> **Note:** The region variable is `LIBSQL_BOTTOMLESS_AWS_DEFAULT_REGION`, not `LIBSQL_BOTTOMLESS_AWS_REGION`.
+> If unset, `sqld` will fail to start with the error:
+> `Error: Internal Error: LIBSQL_BOTTOMLESS_AWS_DEFAULT_REGION was not set`
+
 ### bottomless-cli
 
 Replicated snapshots can be inspected and managed with the official command-line