fix: implement thread-safe in-memory cache in data_loader.py

_projects_cache was declared but load_all_projects() never read or
wrote it, causing a redundant disk read on every request. Added
double-checked locking with threading.Lock so the JSON file is
read once and reused safely across concurrent requests.
clear_cache() now acquires the same lock before resetting.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: anshul23102

utils/data_loader.py

@@ -1,58 +1,58 @@
-# utils/data_loader.py
-# Handles all reading and lookup of project data from the JSON file.
-
 import json
 import os
+import threading
 
-# Build the path to projects.json relative to this file's location
 DATA_FILE = os.path.join(os.path.dirname(__file__), "..", "data", "projects.json")
 
+_projects_cache = None
+_cache_lock = threading.Lock()
+
 
 def load_all_projects():
-    """Read and return the full list of projects from the JSON file."""
-    with open(DATA_FILE, "r", encoding="utf-8") as f:
-        return json.load(f)
+    """Read and return the full list of projects from the JSON file.
+
+    Results are cached in memory after the first read so subsequent calls
+    do not hit the filesystem.
+    """
+    global _projects_cache
+    if _projects_cache is not None:
+        return _projects_cache
+    with _cache_lock:
+        if _projects_cache is None:
+            with open(DATA_FILE, "r", encoding="utf-8") as f:
+                _projects_cache = json.load(f)
+    return _projects_cache
 
 
 def find_project_by_id(project_id):
-    """
-    Return the project dict whose 'id' matches the given integer.
-    Returns None if no match is found.
-    """
+    """Return the project whose 'id' matches project_id, or None."""
     for project in load_all_projects():
         if project.get("id") == project_id:
             return project
     return None
 
+
 def get_project_stats():
-    """
-    Calculate and return statistics about the projects.
-    Returns: { total_projects, unique_skills, beginner_friendly }
-    """
+    """Return total_projects, unique_skills, and beginner_friendly counts."""
     projects = load_all_projects()
-    total_projects = len(projects)
 
-    # Collect all unique skills
     all_skills = set()
+    beginner_friendly = 0
     for p in projects:
         for s in p.get("skills", []):
             all_skills.add(s)
-    unique_skills = len(all_skills)
-
-    # Count beginner projects
-    beginner_friendly = len([p for p in projects if p.get("level") == "Beginner"])
+        if p.get("level") == "Beginner":
+            beginner_friendly += 1
 
     return {
-        "total_projects": total_projects,
-        "unique_skills": unique_skills,
-        "beginner_friendly": beginner_friendly
+        "total_projects": len(projects),
+        "unique_skills": len(all_skills),
+        "beginner_friendly": beginner_friendly,
     }
 
-# Cache for loaded projects
-_projects_cache = None
-
 
 def clear_cache():
-    """Reset the in-memory project cache."""
+    """Reset the in-memory project cache (used in tests)."""
     global _projects_cache
-    _projects_cache = None
+    with _cache_lock:
+        _projects_cache = None