Enforce max_size config with LRU cache eviction

[andrew]

Closes #99.

The max_size storage config was parsed and validated at startup but never enforced. The existing DB queries for LRU eviction (GetLeastRecentlyUsedArtifacts, GetTotalCacheSize, ClearArtifactCache) were already in place but never called.

This adds a background eviction loop that runs every minute when max_size is set. When total cache size exceeds the limit, it evicts least recently used artifacts (deleting from both storage and clearing the DB record) until the cache is back under the limit. When max_size is empty or "0" (unlimited), the loop returns immediately without starting.

Changes:

• config.ParseMaxSize() converts the max_size string to bytes
• server/eviction.go implements the eviction loop and evictLRU function
• server.Start() launches the eviction loop as a background goroutine tied to the existing shutdown context
• Tests cover: under-limit no-op, LRU ordering, multi-artifact eviction, empty cache, storage file cleanup, and unlimited-skips path

[kpfleming]

This looks great!

[kpfleming]

Just upgraded and within a minute my storage amount dropped from 2.7GB to 1.9GB (configured with max_size of 2GB). :-)