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

[anshul23102]

Description

utils/data_loader.py declared _projects_cache = None but load_all_projects() never read or wrote that variable. Every call to load_all_projects() opened and parsed projects.json from disk unconditionally. Routes that touch /, /api/recommend, and /project/<id> each trigger at least one redundant file read per request.

Root Cause

_projects_cache = None  # defined but never checked or populated

def load_all_projects():
    with open(DATA_FILE, "r", encoding="utf-8") as f:
        return json.load(f)  # reads the file on every call

Related Issue

Closes #271

Type of Change

• Bug fix

Changes Made

• utils/data_loader.py:
  • Added _cache_lock = threading.Lock() for thread safety.
  • Implemented double-checked locking in load_all_projects() so the file is read at most once per process lifetime.
  • clear_cache() acquires the lock before resetting _projects_cache = None to prevent partial reads during cache invalidation.

Testing Done

1. First call to load_all_projects(): file is read and result is cached.
2. Subsequent calls: cached list returned without file I/O.
3. clear_cache(): resets the cache; next call re-reads the file.
4. Concurrent calls from multiple threads: no duplicate reads due to the lock.

Checklist

• My code follows the style and formatting of this project
• I have tested my changes locally and they work as expected
• There are no merge conflicts with the base branch
• This PR is linked to the correct issue

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[anshul23102]

@komalharshita this PR is ready for review and all CI checks pass. Could you please add the relevant labels? It helps with tracking. Thank you!

[anshul23102]

Hi @komalharshita, just a gentle check-in on this PR. It has been a couple of days since the last activity. Happy to make any changes if you have feedback. Thanks for your time!

[anshul23102]

Gentle ping -- this PR has been open for 2 days with no activity. Could you please review it when you get a chance? Happy to make any adjustments.

[komalharshita]

Looks safe to merge noq