docs(notification-outbox): retarget S&F notification category to central

docs/requirements/Component-StoreAndForward.md

@@ -13,7 +13,7 @@ Site clusters only. The central cluster does not buffer messages.
 - Buffer outbound messages when the target system is unavailable.
 - Manage three categories of buffered messages:
   - External system API calls.
-  - Email notifications.
+  - Notifications forwarded to the central cluster.
   - Cached database writes.
 - Retry delivery per message according to the configured retry policy.
 - Park messages that exhaust their retry limit (dead-letter).
@@ -42,13 +42,16 @@ Attempt immediate delivery
                     └── Max retries exhausted → Park message
 ```
 
+For notifications, "delivery" means forwarding the message to the central cluster via the Communication Layer; "success" is central's ack, on which the message is cleared. Notifications do not park — they are retried at the fixed forward interval until central acks. Parking applies only to the external-system-call and cached-database-write categories.
+
 ## Retry Policy
 
-Retry settings are defined on the **source entity** (not per-message):
+For the external-system-call and cached-database-write categories, retry settings are defined on the **source entity** (not per-message):
 - **External systems**: Each external system definition includes max retry count and time between retries.
-- **Notifications**: Email/SMTP configuration includes max retry count and time between retries.
 - **Cached database writes**: Each database connection definition includes max retry count and time between retries.
 
+The **notification** category retries differently: it has no source-entity setting. The site→central forward uses a single fixed retry interval configured in the host `appsettings.json`. This interval concerns reaching the central cluster — it is infrastructure config, not a per-notification-list setting — so it applies uniformly to every buffered notification regardless of its target list. A buffered notification is retried until central acks it; it is not parked on a retry limit (central, once reachable, owns delivery, retry, and parking from that point on).
+
 The retry interval is **fixed** (not exponential backoff). Fixed interval is sufficient for the expected use cases.
 
 **Note**: Only **transient failures** are eligible for store-and-forward buffering. For external system calls, transient failures are connection errors, timeouts, and HTTP 5xx responses. Permanent failures (HTTP 4xx) are returned directly to the calling script and are **not** queued for retry. This prevents the buffer from accumulating requests that will never succeed.
@@ -79,8 +82,8 @@ There is **no maximum buffer size**. Messages accumulate in the buffer until del
 Each buffered message stores:
 - **Message ID**: Unique identifier.
 - **Category**: External system call, notification, or cached database write.
-- **Target**: External system name, notification list name, or database connection name.
+- **Target**: External system name, the central cluster (for notifications), or database connection name.
-- **Payload**: Serialized message content (API method + parameters, email subject + body, SQL + parameters).
+- **Payload**: Serialized message content (API method + parameters; notification list name + subject + body plus the locally generated `NotificationId` and source provenance; SQL + parameters).
 - **Retry Count**: Number of attempts so far.
 - **Created At**: Timestamp when the message was first queued.
 - **Last Attempt At**: Timestamp of the most recent delivery attempt.
@@ -89,14 +92,15 @@ Each buffered message stores:
 ## Dependencies
 
 - **SQLite**: Local persistence on each node.
-- **Communication Layer**: Application-level replication to standby node; remote query handling from central.
+- **Communication Layer**: Application-level replication to standby node; remote query handling from central; carries buffered notifications to the central cluster (ClusterClient) and receives central's acks.
 - **External System Gateway**: Delivers external system API calls.
-- **Notification Service**: Delivers email notifications.
+- **Central–Site Communication**: The delivery target for the notification category — a buffered notification is forwarded to the central cluster over the Communication Layer and cleared on central's ack.
 - **Database Connections**: Delivers cached database writes.
 - **Site Event Logging**: Logs store-and-forward activity (queued, delivered, retried, parked).
 
 ## Interactions
 
 - **Site Runtime (Script Actors)**: Scripts submit messages to the buffer (external calls, notifications, cached DB writes).
-- **Communication Layer**: Handles parked message queries/commands from central.
+- **Communication Layer**: Handles parked message queries/commands from central; carries buffered notifications to the central cluster.
-- **Health Monitoring**: Reports buffer depth metrics.
+- **Notification Outbox**: The central destination for the notification category — central ingests each forwarded notification into the `Notifications` table and acks the site, on which the engine clears the buffered message.
+- **Health Monitoring**: Reports buffer depth metrics, including the notification backlog covering the site→central forward leg.