Implement Persistent Version Metadata Cache with Security Vulnerability Indexing

[ShaikhWarsi]

Fixes #71

PR Summary

• Adds a local SQLite metadata cache with 24h TTL for Python releases/versions.

• Implements a background sync worker for python.org; cache-first reads with async refresh.

• Overhauls version fetching to use the cache; adds filtering API and security-support helper.

• Improves TUI responsiveness by avoiding blocking network calls.

• Prepares groundwork for CVE ingestion and additional sources (conda-forge) in future PRs.
  Key Changes

• New: metadata_store.py (SQLite schema, TTL checks, sync + background worker)

• Update: version.py (cache integration, advanced filtering, security/EOL helpers)

• Update: constants.py (METADATA_DB, METADATA_TTL_SECONDS)
  Impact

• Faster, reliable version data in TUI/CLI with offline support.

• Consistent status normalization for EOL/security warnings.

• Extensible path for multi-source metadata and CVE integration

[ShaikhWarsi]

Only a few hours left before acwoc ends , please merge this pr before that @shreyasmene06

[shreyasmene06]

Excellent implementation of the metadata cache system! This is a substantial, well-designed feature.

What's great

• SQLite-based persistent cache with proper schema
• TTL logic with 24h default, configurable via constants
• Non-blocking background sync using threading
• Thread-safe with threading.Lock()
• Graceful fallback to network when cache is empty
• EOL/security status tracking
• Advanced filtering API (get_versions_filtered, is_version_security_supported)
• Clean integration with existing version.py

Suggestions (non-blocking)

1. Add tests

This is a complex feature that really should have tests. Consider adding:

# tests/test_metadata_store.py
def test_cache_creation():
    """Test that cache DB is created."""
def test_is_cache_stale_when_empty():
    """Test stale detection on fresh DB."""
def test_sync_populates_cache():
    """Test that sync adds data to cache."""
def test_background_sync_is_non_blocking():
    """Test that background sync doesn't block."""
def test_get_versions_filtered():
    """Test filtering by min_version and security status."""

2. Consider adding CLI commands (follow-up PR)
   pyvm cache status # Show cache age, entry count
   pyvm cache refresh # Force sync
   pyvm cache clear # Delete cache
3. Minor type hint

This is exactly what issue #71 requested - a robust local data engine with caching, EOL tracking, and filtering. Great work on a hard issue!
LGTM! 🎉