Address code review feedback

- Fixed status filtering to occur before pagination for correct counts
- Resolved N+1 query problem by batch-fetching processing statuses
- Extracted status computation logic to shared utility function
- Changed sort indicator from ⬍ to ↕ for better browser compatibility
- Updated both API and view layers to use shared status utilities

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/files.html

@@ -308,7 +308,7 @@
               {% if sort_by == 'id' %}
                 {% if sort_order == 'asc' %}▲{% else %}▼{% endif %}
               {% else %}
-                ⬍
+                ↕
               {% endif %}
             </span>
           </th>
@@ -318,7 +318,7 @@
               {% if sort_by == 'original_filename' %}
                 {% if sort_order == 'asc' %}▲{% else %}▼{% endif %}
               {% else %}
-                ⬍
+                ↕
               {% endif %}
             </span>
           </th>
@@ -328,7 +328,7 @@
               {% if sort_by == 'file_size' %}
                 {% if sort_order == 'asc' %}▲{% else %}▼{% endif %}
               {% else %}
-                ⬍
+                ↕
               {% endif %}
             </span>
           </th>
@@ -338,7 +338,7 @@
               {% if sort_by == 'mime_type' %}
                 {% if sort_order == 'asc' %}▲{% else %}▼{% endif %}
               {% else %}
-                ⬍
+                ↕
               {% endif %}
             </span>
           </th>
@@ -349,7 +349,7 @@
               {% if sort_by == 'created_at' %}
                 {% if sort_order == 'asc' %}▲{% else %}▼{% endif %}
               {% else %}
-                ⬍
+                ↕
               {% endif %}
             </span>
           </th>