Fix: Search Index Stale After Post Deletion

[Techhackontime999]

Description

Deleted posts continue to appear in search results until the search cache expires naturally (which can be up to 1 hour).

Steps to Reproduce

1. Create a post with unique content
2. Search for that content - finds the post
3. Delete the post
4. Search again immediately - the deleted post still appears
5. Wait for cache to expire - post disappears

Expected Behavior

Deleted posts should be immediately removed from search results.

Root Cause

The search cache (Redis) stores results by query hash, and deleting a post doesn't invalidate the relevant search cache entries.

Files to Fix

• linkup/feed/views.py - On post delete, invalidate search cache entries
• linkup/core/views.py - Add cache invalidation method for search results
• linkup/core/performance.py - Add targeted cache invalidation

Acceptance Criteria

• Deleted posts disappear from search immediately
• Cache invalidation doesn't affect unrelated cached queries
• Search performance is not degraded by invalidation