feat: global email body cache eviction

[FromSi]

Closes #521

What?

Adds a configurable global size limit for cached email bodies.

• Defaults body_cache_threshold_mb to 100 MB.
• Applies the limit across the whole ~/.cache/matcha/email_bodies/ directory, not per folder cache file.
• Evicts least recently accessed cached bodies until the total cache size is back under the limit.
• Skips caching bodies larger than the configured limit.
• Adds tests for cross-folder LRU eviction, multi-entry eviction, oversized replacements, and the default threshold.
• Updates the seed script to generate 10 KB email bodies for cache testing.

Why?

Email body cache files could grow without a global cap for active users with large mailboxes. Existing pruning only removed bodies for emails no longer present in a folder, so valid cached emails could accumulate indefinitely. A global LRU limit keeps disk usage bounded while preserving recently used cached messages.

[floatpanebot]

Hi @FromSi! Please fix the following issues with your PR:

• Title: Is too long (42 characters). The PR title must be strictly under 40 characters.

Formatting issues have been resolved. Thank you!