feat: Add cache for all filters

[shouya]

Background

In PR #112, @olehbozhok implemented caching for the full_text filter, significantly boosting performance on resource-constrained devices. This improvement is particularly effective for filters requiring network fetches or HTML processing.

Current Implementation

This PR extends the caching behavior to all filters, with two levels of cache granularity:

1. Feed-level cache (default):

   • Skips the filter if the input feed is found in the cache
   • Expected to be effective due to infrequent feed changes between fetches

2. Post-level cache (opt-in):

   • Caches individual posts
   • Run the filter on a modified feed with posts not found in the cache
   • Correctness depends on filter implementation, thus opt-in by certain filters only
   • Feed-level cache is still checked first

Both cache levels use expiring LRU (Least Recently Used) caches with hard expiration times:

• Feed-level: size=5, TTL=12 hours
• Post-level: size=40, TTL=1 hour

Additional Features

• Cache parameters are currently hard-coded but can be made configurable upon request
• Caches persist across hot-reloads due to config changes
  • Only caches for changed filters are invalidated during hot-reloads