feat: add attachments endpoint for contact media view (#14391)

# Pull Request Template

## Description

This PR adds an endpoint to fetch all attachments shared with or by a
contact across all of their conversations.

Results are scoped based on the access:
* Admins can access all attachments
* Agents can access attachments only from inboxes they belong to
* Custom role agents are further filtered based on their conversation
permissions

Each attachment payload includes `conversation_id`, allowing the UI to
deep-link back to the source conversation.

Added `GET
/api/v1/accounts/:account_id/contacts/:contact_id/attachments` under the
existing contacts scope.

Fixes
https://linear.app/chatwoot/issue/CW-7021/add-media-view-to-the-contact-details-page

## Type of change

- [x] New feature (non-breaking change which adds functionality)


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/controllers/api/v1/accounts/contacts/attachments_controller.rb

@@ -0,0 +1,18 @@
+class Api::V1::Accounts::Contacts::AttachmentsController < Api::V1::Accounts::Contacts::BaseController
+  RESULTS_PER_PAGE = 100
+
+  def index
+    conversations = Conversations::PermissionFilterService.new(
+      Current.account.conversations.where(contact_id: @contact.id),
+      Current.user,
+      Current.account
+    ).perform
+
+    @attachments = Attachment.where(message_id: Message.where(conversation_id: conversations).select(:id))
+                             .includes({ file_attachment: :blob }, message: [:conversation, :inbox, { sender: { avatar_attachment: :blob } }])
+                             .order(created_at: :desc)
+                             .page(params[:page])
+                             .per(RESULTS_PER_PAGE)
+    @attachments_count = @attachments.total_count
+  end
+end

app/controllers/api/v1/accounts/conversations_controller.rb

@@ -28,7 +28,7 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
   def attachments
     @attachments_count = @conversation.attachments.count
     @attachments = @conversation.attachments
-                                .includes(:message)
+                                .includes({ file_attachment: :blob }, message: [:inbox, { sender: { avatar_attachment: :blob } }])
                                 .order(created_at: :desc)
                                 .page(attachment_params[:page])
                                 .per(ATTACHMENT_RESULTS_PER_PAGE)