DAS_KMS_REGION_NAME documented in README but never used in code

[ngoyal16]

Problem

The DAS_KMS_REGION_NAME environment variable is documented in README.md but never used anywhere in the codebase.

Variable	Default	Description
DAS_KMS_REGION_NAME		Region where the KMS key resides

Searching main.go, the only env vars read are:

• DAS_FILTER_NAME (line 69)
• DAS_RDS_RESOURCE_ID (line 79)

DAS_KMS_REGION_NAME is not referenced. This means:

1. If the KMS key is in a different region than the Lambda execution environment, decryption will fail — there is no way to configure a KMS client with a custom region.
2. The README is misleading — operators may set this variable expecting it to work, but it has no effect.

Proposed Solution

Option A: Wire it up (preferred)

Use DAS_KMS_REGION_NAME to initialize the KMS client with a specific region:

kmsRegion := os.Getenv("DAS_KMS_REGION_NAME")
var kmsClient *kms.Client
if kmsRegion != "" {
    kmsCfg, err := config.LoadDefaultConfig(ctx, config.WithRegion(kmsRegion))
    // ...
    kmsClient = kms.NewFromConfig(kmsCfg)
} else {
    kmsClient = kms.NewFromConfig(cfg)
}

Option B: Remove it

If the KMS key is always in the same region as the Lambda, remove DAS_KMS_REGION_NAME from the README to avoid confusion.

Acceptance Criteria

• Either the env var is used in code, or it is removed from documentation
• If wired up: KMS client respects the configured region
• README accurately reflects all environment variables used by the code