perf(web): use content hash for static asset cache busting

Switch from timestamp-based to MD5 content hash versioning.
Cache now only invalidates when file content actually changes.

src/server/utils/template_helpers.py

@@ -13,8 +13,8 @@ Series Identifier Convention:
 All template helpers that handle series data use `key` for identification and
 provide `folder` as display metadata only.
 """
+import hashlib
 import logging
-import time
 from pathlib import Path
 from typing import Any, Dict, List, Optional
 
@@ -27,10 +27,44 @@ logger = logging.getLogger(__name__)
 
 # Configure templates directory
 TEMPLATES_DIR = Path(__file__).parent.parent / "web" / "templates"
+STATIC_DIR = Path(__file__).parent.parent / "web" / "static"
 templates = Jinja2Templates(directory=str(TEMPLATES_DIR))
 
-# Version token for static asset cache-busting; changes on every server start.
-STATIC_VERSION: str = str(int(time.time()))
+# Cache for static file hashes: {file_path: (mtime, hash)}
+_hash_cache: Dict[str, tuple[float, str]] = {}
+
+
+def get_static_version(file_path: str) -> str:
+    """
+    Get cache-busting version for a static file based on content hash.
+    
+    Hash is computed once and cached; cache is invalidated when file mtime changes.
+    
+    Args:
+        file_path: Relative path to static file (e.g., 'css/styles.css')
+        
+    Returns:
+        8-character hex hash of file content, or empty string if file not found
+    """
+    full_path = STATIC_DIR / file_path
+    
+    if not full_path.exists():
+        logger.warning(f"Static file not found: {file_path}")
+        return ""
+    
+    current_mtime = full_path.stat().st_mtime
+    
+    # Check cache validity
+    if file_path in _hash_cache:
+        cached_mtime, cached_hash = _hash_cache[file_path]
+        if cached_mtime == current_mtime:
+            return cached_hash
+    
+    # Compute new hash
+    file_hash = hashlib.md5(full_path.read_bytes()).hexdigest()[:8]
+    _hash_cache[file_path] = (current_mtime, file_hash)
+    
+    return file_hash
 
 
 def get_base_context(
@@ -51,7 +85,7 @@ def get_base_context(
         "title": title,
         "app_name": "Aniworld Download Manager",
         "version": APP_VERSION,
-        "static_v": STATIC_VERSION,
+        "static_version": get_static_version,
     }