fix(po): grant KMS on seahaven-dynamodb CMK to purchase-orders consumers (INFRA-104) (#56)

The purchase-orders table was migrated to SSE-KMS (alias/seahaven-dynamodb,
INFRA-95/M-3) out-of-band, but po_stack never declared the key, so
grant_read_write_data did not propagate kms perms. po-email-processor failed
~99.6% of invocations with kms:Decrypt AccessDeniedException, a data-loss
outage on the PO ingestion write path.

- po_stack: declare encryption_key on purchase-orders (reconciles SSE drift;
  no-op against the already-encrypted live table) so the existing grants add
  kms:Decrypt/GenerateDataKey/DescribeKey to EmailProcessor, WebUI, SiteExtractor.
- wo_stack: pre-emptive grant_encrypt_decrypt on the WO processor role ahead of
  the WorkOrders CMK migration (INFRA-6); tables left unencrypted, no table change.

GPT-4.1 cross-review: no blockers.

cdk/po_stack.py

@@ -8,6 +8,7 @@ from aws_cdk import (
     aws_cloudwatch as cloudwatch,
     aws_cloudwatch_actions as cw_actions,
     aws_dynamodb as dynamodb,
+    aws_kms as kms,
     aws_lambda as lambda_,
     aws_lambda_event_sources as lambda_event_sources,
     aws_logs as logs,
@@ -18,6 +19,7 @@ from aws_cdk import (
     aws_secretsmanager as secretsmanager,
     aws_sns as sns,
     aws_sqs as sqs,
+    aws_ssm as ssm,
 )
 from constructs import Construct
 
@@ -37,6 +39,20 @@ class PoIngestStack(Stack):
             ],
         )
 
+        # --- Shared customer-managed CMK for sensitive DynamoDB tables ---
+        # Owned by the account-baseline app (alias/seahaven-dynamodb, INFRA-95 /
+        # M-3); ARN published to SSM. The purchase-orders table was migrated to
+        # SSE-KMS out-of-band, so declaring encryption_key here reconciles the
+        # drift and — via grant_read_write_data below — propagates the required
+        # kms:Decrypt/GenerateDataKey/DescribeKey to the consumer roles.
+        dynamodb_cmk = kms.Key.from_key_arn(
+            self,
+            "DynamoDbCmk",
+            ssm.StringParameter.value_for_string_parameter(
+                self, "/seahaven/dynamodb/cmk-arn"
+            ),
+        )
+
         # --- Purchase-orders DynamoDB table ---
         # Owned by this stack. Streams enabled for the site-extractor pipeline.
         # Other stacks (seahaven-slack-bot) reference this table via fromTableName().
@@ -51,6 +67,8 @@ class PoIngestStack(Stack):
             billing_mode=dynamodb.BillingMode.PAY_PER_REQUEST,
             removal_policy=RemovalPolicy.RETAIN,
             stream=dynamodb.StreamViewType.NEW_IMAGE,
+            encryption=dynamodb.TableEncryption.CUSTOMER_MANAGED,
+            encryption_key=dynamodb_cmk,
         )
 
         # --- Secrets Manager for Anthropic API key ---

cdk/wo_stack.py

@@ -8,6 +8,7 @@ from aws_cdk import (
     aws_cloudwatch as cloudwatch,
     aws_cloudwatch_actions as cw_actions,
     aws_dynamodb as dynamodb,
+    aws_kms as kms,
     aws_lambda as lambda_,
     aws_logs as logs,
     aws_s3 as s3,
@@ -17,6 +18,7 @@ from aws_cdk import (
     aws_secretsmanager as secretsmanager,
     aws_sns as sns,
     aws_sqs as sqs,
+    aws_ssm as ssm,
 )
 from constructs import Construct
 
@@ -127,6 +129,22 @@ class WorkorderIngestStack(Stack):
         comments_table.grant_read_write_data(email_processor)
         anthropic_secret.grant_read(email_processor)
 
+        # Pre-emptive KMS grant on the shared DynamoDB CMK (alias/seahaven-dynamodb,
+        # /seahaven/dynamodb/cmk-arn). The WorkOrders/WorkOrderComments tables are
+        # NOT yet SSE-KMS encrypted, so this grant is currently unused; it is added
+        # ahead of the CMK migration so the processor role does not hit AccessDenied
+        # the moment those tables are migrated. The actual table migration + kebab
+        # rename is tracked in INFRA-6 (and the set-aside wo_stack CMK WIP); fold the
+        # web-ui read grant in there.
+        dynamodb_cmk = kms.Key.from_key_arn(
+            self,
+            "DynamoDbCmk",
+            ssm.StringParameter.value_for_string_parameter(
+                self, "/seahaven/dynamodb/cmk-arn"
+            ),
+        )
+        dynamodb_cmk.grant_encrypt_decrypt(email_processor)
+
         # --- Errors alarm (INFRA-41 / audit H-8) ---
         # ALARM-only (no OK action, per the CloudWatch-alarm preference) to the
         # shared site-alerts topic (CMK alias/seahaven-alarm-topics lives on the