feat(applications): add DELETE endpoint for preview deployments by PR id

Add `DELETE /api/v1/applications/{uuid}/previews/{pull_request_id}` to
cancel active deployments, stop containers, and delete the preview
record via `CleanupPreviewDeployment`. Includes OpenAPI annotations,
input validation, and full feature test coverage.

openapi.yaml

@@ -2398,6 +2398,48 @@ paths:
       security:
         -
           bearerAuth: []
+  '/applications/{uuid}/previews/{pull_request_id}':
+    delete:
+      tags:
+        - Applications
+      summary: 'Delete Preview Deployment'
+      description: 'Delete a preview deployment for a pull request. Cancels active deployments, stops containers, removes volumes/networks, and deletes the preview record.'
+      operationId: delete-preview-deployment-by-pull-request-id
+      parameters:
+        -
+          name: uuid
+          in: path
+          description: 'UUID of the application.'
+          required: true
+          schema:
+            type: string
+        -
+          name: pull_request_id
+          in: path
+          description: 'Pull request ID of the preview to delete.'
+          required: true
+          schema:
+            type: integer
+      responses:
+        '200':
+          description: 'Preview deletion queued.'
+          content:
+            application/json:
+              schema:
+                properties:
+                  message: { type: string }
+                type: object
+        '401':
+          $ref: '#/components/responses/401'
+        '400':
+          $ref: '#/components/responses/400'
+        '404':
+          $ref: '#/components/responses/404'
+        '422':
+          $ref: '#/components/responses/422'
+      security:
+        -
+          bearerAuth: []
   /cloud-tokens:
     get:
       tags: