Feed health dashboard CLI command

[ArkNill]

Feature Request

Add feedkit health command showing feed reliability statistics:

$ feedkit health
Feed                        Success  Errors  Last OK         Status
BBC Top Stories             45/45    0       2 hours ago     healthy
arXiv cs.AI                 43/45    2       1 day ago       degraded
Broken Feed                 0/10     10      never           dead

Implementation

Data already exists in FeedStore.get_health(). Just needs a CLI command with rich table formatting.