Fix Data retention rules added

DATA_RETENTION.md

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+# Data Retention Rules
+
+This document defines the default retention policy for records that are stale, inactive, or no longer useful in the TaskBounty application.
+
+## Scope
+
+The retention rules apply to persistent in-memory records used by the application runtime, including:
+
+- Notifications created for user activity or platform broadcasts
+- Completed, cancelled, or disputed task records
+- Related submission and comment records that belong to expired task records
+
+## Categories and retention periods
+
+| Record category | Status / trigger | Retention period | Safe handling |
+|---|---|---:|---|
+| User notifications | Created and left unread/unused beyond the configured expiry window | 30 days | Delete lazily during read/write operations and when listing notifications |
+| Completed task records | Finalized tasks older than the retention window | 180 days | Remove task, submissions, and comments tied to the task after expiry |
+| Cancelled task records | Finalized tasks older than the retention window | 180 days | Remove the task and any dependent records after expiry |
+| Disputed task records | Finalized tasks older than the retention window | 180 days | Remove the task and related discussion data after expiry |
+
+## Configuration
+
+Retention values are configured through environment variables so deployments can tune policy without changing application code:
+
+- `NEXT_PUBLIC_NOTIFICATION_RETENTION_DAYS` — default `30`
+- `NEXT_PUBLIC_TASK_RETENTION_DAYS` — default `180`
+
+If the environment variable is missing or invalid, the defaults above are used.
+
+## Expired record handling
+
+Expired records are removed safely in a lazy cleanup pass before creates, reads, and listing operations. This approach avoids hard-failing operations when old data is present while ensuring stale records do not accumulate indefinitely.
+
+The cleanup process does the following:
+
+1. Identifies records older than the configured retention window.
+2. Removes only inactive or finalized records eligible for deletion.
+3. Deletes dependent child records (e.g., task submissions/comments) when the parent task is expired.
+4. Keeps active or still-valid records intact.
+
+## Operational guidance
+
+- Prefer a short retention period for notifications to avoid indefinite in-memory growth.
+- Keep task retention longer than notifications because historical task data may be useful for auditing and review.
+- If a stricter compliance requirement is introduced later, update this document and the corresponding environment values together.