fix: reload asset tag files in DEBUG mode when modified

Add mtime tracking to asset tag file cache to automatically reload
files during development when they change. In production, maintain
permanent in-memory cache for optimal performance.

Changes:
- Track file mtime and path in cache structure alongside content
- Check mtime in DEBUG mode and invalidate cache when file changes
- Skip mtime checks in production for better performance
- Extract _empty_cache_entry helper to avoid duplication
- Early return in production mode for faster cache hits

This fixes the issue where rebuilt asset files weren't reflected
without restarting the Django server during development.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: dragonee

tasks/apps/common/templatetags/render_assets.py

@@ -7,6 +7,7 @@
 register = template.Library()
 
 # In-memory caches for CSS and JS tag files
+# Structure: {entry_point: {'content': str, 'mtime': float, 'path': Path}}
 CSS_FILES = {}
 JS_FILES = {}
 
@@ -29,20 +30,26 @@ def get_potential_staticfile_paths(filename):
         for static_dir in settings.STATICFILES_DIRS:
             yield Path(static_dir) / filename
 
+def _empty_cache_entry():
+    return {'content': '', 'mtime': None, 'path': None}
 
 def read_cached_staticfile(entry_point, file_type, cache):
     """Reads and caches static tag files.
 
+    In DEBUG mode, checks file mtime and reloads if changed.
+    In production, uses permanent in-memory cache.
+
     Args:
         entry_point - Name of the entry point (e.g., 'app', 'hello_world_mount')
         file_type - Type of file ('css' or 'js')
         cache - Dictionary to use for caching
     Returns:
         HTML content from the cached file or empty string if not found.
     """
-    # Check if already cached
-    if entry_point in cache:
-        return cache[entry_point]
+
+    # In production, use permanent in-memory cache
+    if not settings.DEBUG and entry_point in cache:
+        return cache[entry_point]['content']
 
     filename = f'{entry_point}-{file_type}-tags.html'
 
@@ -54,17 +61,38 @@ def read_cached_staticfile(entry_point, file_type, cache):
 
     if html_file_path is None:
         # No file found, cache empty string
-        cache[entry_point] = ''
+        if entry_point not in cache:
+            cache[entry_point] = _empty_cache_entry()
         return ''
 
     try:
+        # In DEBUG mode, check if file has been modified
+        if settings.DEBUG and entry_point in cache:
+            cached_mtime = cache[entry_point].get('mtime')
+            current_mtime = html_file_path.stat().st_mtime
+
+            # If mtime changed, invalidate cache
+            if cached_mtime is not None and cached_mtime != current_mtime:
+                del cache[entry_point]
+
+        # Check if already cached (and still valid)
+        if entry_point in cache:
+            return cache[entry_point]['content']
+
+        # Read and cache the content
         content = html_file_path.read_text()
-        # Cache the content
-        cache[entry_point] = content
+        mtime = html_file_path.stat().st_mtime if settings.DEBUG else None
+
+        cache[entry_point] = {
+            'content': content,
+            'mtime': mtime,
+            'path': html_file_path
+        }
         return content
     except (FileNotFoundError, OSError):
         # Cache empty string to avoid repeated file lookups
-        cache[entry_point] = ''
+        if entry_point not in cache:
+            cache[entry_point] = _empty_cache_entry()
         return ''